Merge remote-tracking branch 'origin/master' into NumericUpDown_Fix_5318

# Conflicts:
#	src/Avalonia.Controls/ApiCompatBaseline.txt

.gitignore

@@ -106,6 +106,9 @@ _NCrunch_*/
 *.ncrunchsolution.user
 nCrunchTemp_*
 
+# CodeRush
+.cr/
+
 # Others
 sql/
 *.Cache

native/Avalonia.Native/inc/comimpl.h

@@ -8,8 +8,109 @@
 
 #include <cstring>
 
+/**
+ START_COM_CALL causes AddRef to be called at the beggining of a function.
+ When a function is exited, it causes ReleaseRef to be called.
+ This ensures that the object cannot be destroyed whilst the function is running.
+ For example: Window Show is called, which triggers an event, and user calls Close inside the event
+ causing the refcount to reach 0, and the object to be destroyed. Function then continues and this pointer
+ will now be invalid.
+ 
+ START_COM_CALL protects against this scenario.
+ */
+#define START_COM_CALL auto r = this->UnknownSelf()
+
 __IID_DEF(IUnknown, 0, 0, 0, C0, 00, 00, 00, 00, 00, 00, 46);
 
+template<class TInterface>
+class ComPtr
+{
+private:
+    TInterface* _obj;
+public:
+    ComPtr()
+    {
+        _obj = 0;
+    }
+    
+    ComPtr(TInterface* pObj)
+    {
+        _obj = 0;
+
+        if (pObj)
+        {
+            _obj = pObj;
+            _obj->AddRef();
+        }
+    }
+    
+    ComPtr(const ComPtr& ptr)
+    {
+        _obj = 0;
+        
+        if (ptr._obj)
+        {
+            _obj = ptr._obj;
+            _obj->AddRef();
+        }
+
+    }
+    
+    ComPtr& operator=(ComPtr other)
+    {
+        if(_obj != NULL)
+            _obj->Release();
+        _obj = other._obj;
+        if(_obj != NULL)
+            _obj->AddRef();
+        return *this;
+    }
+
+    ~ComPtr()
+    {
+        if (_obj)
+        {
+            _obj->Release();
+            _obj = 0;
+        }
+    }
+    
+    TInterface* getRaw()
+    {
+        return _obj;
+    }
+    
+    TInterface* getRetainedReference()
+    {
+        if(_obj == NULL)
+            return NULL;
+        _obj->AddRef();
+        return _obj;
+    }
+    
+    TInterface** getPPV()
+    {
+        return &_obj;
+    }
+    
+    operator TInterface*() const
+    {
+        return _obj;
+    }
+    TInterface& operator*() const
+    {
+        return *_obj;
+    }
+    TInterface** operator&()
+    {
+        return &_obj;
+    }
+    TInterface* operator->() const
+    {
+        return _obj;
+    }
+};
+
 class ComObject : public virtual IUnknown
 {
 private:
@@ -58,6 +159,12 @@ public:
         _refCount++;
         return S_OK;
     }
+    
+protected:
+   ComPtr<IUnknown> UnknownSelf()
+    {
+       return this;
+   }
 
 };
 
@@ -104,94 +211,5 @@ public:
     virtual ~ComSingleObject(){}
 };
 
-template<class TInterface>
-class ComPtr
-{
-private:
-    TInterface* _obj;
-public:
-    ComPtr()
-    {
-        _obj = 0;
-    }
-    
-    ComPtr(TInterface* pObj)
-    {
-        _obj = 0;
-
-        if (pObj)
-        {
-            _obj = pObj;
-            _obj->AddRef();
-        }
-    }
-    
-    ComPtr(const ComPtr& ptr)
-    {
-        _obj = 0;
-        
-        if (ptr._obj)
-        {
-            _obj = ptr._obj;
-            _obj->AddRef();
-        }
-
-    }
-    
-    ComPtr& operator=(ComPtr other)
-    {
-        if(_obj != NULL)
-            _obj->Release();
-        _obj = other._obj;
-        if(_obj != NULL)
-            _obj->AddRef();
-        return *this;
-    }
-
-    ~ComPtr()
-    {
-        if (_obj)
-        {
-            _obj->Release();
-            _obj = 0;
-        }
-    }
-    
-    TInterface* getRaw()
-    {
-        return _obj;
-    }
-    
-    TInterface* getRetainedReference()
-    {
-        if(_obj == NULL)
-            return NULL;
-        _obj->AddRef();
-        return _obj;
-    }
-    
-    TInterface** getPPV()
-    {
-        return &_obj;
-    }
-    
-    operator TInterface*() const
-    {
-        return _obj;
-    }
-    TInterface& operator*() const
-    {
-        return *_obj;
-    }
-    TInterface** operator&()
-    {
-        return &_obj;
-    }
-    TInterface* operator->() const
-    {
-        return _obj;
-    }
-};
-
 #endif // COMIMPL_H_INCLUDED
 #pragma clang diagnostic pop

native/Avalonia.Native/src/OSX/AvnString.mm

@@ -43,6 +43,8 @@ public:
     
     virtual HRESULT Pointer(void**retOut) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(retOut == nullptr)
@@ -58,14 +60,19 @@ public:
     
     virtual HRESULT Length(int*retOut) override
     {
-        if(retOut == nullptr)
+        START_COM_CALL;
+        
+        @autoreleasepool
         {
-            return E_POINTER;
+            if(retOut == nullptr)
+            {
+                return E_POINTER;
+            }
+            
+            *retOut = _length;
+            
+            return S_OK;
         }
-        
-        *retOut = _length;
-        
-        return S_OK;
     }
 };
 
@@ -109,10 +116,15 @@ public:
     
     virtual HRESULT Get(unsigned int index, IAvnString**ppv) override
     {
-        if(_list.size() <= index)
-            return E_INVALIDARG;
-        *ppv = _list[index].getRetainedReference();
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            if(_list.size() <= index)
+                return E_INVALIDARG;
+            *ppv = _list[index].getRetainedReference();
+            return S_OK;
+        }
     }
 };
 

native/Avalonia.Native/src/OSX/Screens.mm

@@ -8,6 +8,8 @@ class Screens : public ComSingleObject<IAvnScreens, &IID_IAvnScreens>
 public:
     virtual HRESULT GetScreenCount (int* ret) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             *ret = (int)[NSScreen screens].count;
@@ -18,6 +20,8 @@ public:
     
     virtual HRESULT GetScreen (int index, AvnScreen* ret) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(index < 0 || index >= [NSScreen screens].count)

native/Avalonia.Native/src/OSX/cgl.mm

@@ -69,6 +69,8 @@ public:
     
     virtual HRESULT LegacyMakeCurrent() override
     {
+        START_COM_CALL;
+        
         if(CGLSetCurrentContext(Context) != 0)
             return E_FAIL;
         return S_OK;
@@ -76,6 +78,8 @@ public:
     
     virtual HRESULT MakeCurrent(IUnknown** ppv) override
     {
+        START_COM_CALL;
+        
         CGLContextObj saved = CGLGetCurrentContext();
         CGLLockContext(Context);
         if(CGLSetCurrentContext(Context) != 0)
@@ -128,6 +132,8 @@ public:
     
     virtual HRESULT CreateContext(IAvnGlContext* share, IAvnGlContext**ppv) override
     {
+        START_COM_CALL;
+        
         CGLContextObj shareContext = nil;
         if(share != nil)
         {
@@ -144,6 +150,8 @@ public:
     
     virtual HRESULT WrapContext(void* native, IAvnGlContext**ppv) override
     {
+        START_COM_CALL;
+        
         if(native == nil)
             return E_INVALIDARG;
         *ppv = new AvnGlContext((CGLContextObj) native);

native/Avalonia.Native/src/OSX/clipboard.mm

@@ -25,6 +25,8 @@ public:
    
     virtual HRESULT GetText (char* type, IAvnString**ppv) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(ppv == nullptr)
@@ -42,6 +44,8 @@ public:
     
     virtual HRESULT GetStrings(char* type, IAvnStringArray**ppv) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             *ppv= nil;
@@ -69,56 +73,71 @@ public:
     
     virtual HRESULT SetText (char* type, char* utf8String) override
     {
-        Clear();
+        START_COM_CALL;
+        
         @autoreleasepool
         {
+            Clear();
+            
             auto string = [NSString stringWithUTF8String:(const char*)utf8String];
             auto typeString = [NSString stringWithUTF8String:(const char*)type];
             if(_item == nil)
                 [_pb setString: string forType: typeString];
             else
                 [_item setString: string forType:typeString];
-        }
         
-        return S_OK;
+            return S_OK;
+        }
     }
     
     virtual HRESULT SetBytes(char* type, void* bytes, int len) override
     {
-        auto typeString = [NSString stringWithUTF8String:(const char*)type];
-        auto data = [NSData dataWithBytes:bytes length:len];
-        if(_item == nil)
-            [_pb setData:data forType:typeString];
-        else
-            [_item setData:data forType:typeString];
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            auto typeString = [NSString stringWithUTF8String:(const char*)type];
+            auto data = [NSData dataWithBytes:bytes length:len];
+            if(_item == nil)
+                [_pb setData:data forType:typeString];
+            else
+                [_item setData:data forType:typeString];
+            return S_OK;
+        }
     }
        
     virtual HRESULT GetBytes(char* type, IAvnString**ppv) override
     {
-        *ppv = nil;
-        auto typeString = [NSString stringWithUTF8String:(const char*)type];
-        NSData*data;
-        @try
+        START_COM_CALL;
+        
+        @autoreleasepool
         {
-            if(_item)
-                data = [_item dataForType:typeString];
-            else
-                data = [_pb dataForType:typeString];
-            if(data == nil)
+            *ppv = nil;
+            auto typeString = [NSString stringWithUTF8String:(const char*)type];
+            NSData*data;
+            @try
+            {
+                if(_item)
+                    data = [_item dataForType:typeString];
+                else
+                    data = [_pb dataForType:typeString];
+                if(data == nil)
+                    return E_FAIL;
+            }
+            @catch(NSException* e)
+            {
                 return E_FAIL;
+            }
+            *ppv = CreateByteArray((void*)data.bytes, (int)data.length);
+            return S_OK;
         }
-        @catch(NSException* e)
-        {
-            return E_FAIL;
-        }
-        *ppv = CreateByteArray((void*)data.bytes, (int)data.length);
-        return S_OK;
     }
 
 
     virtual HRESULT Clear() override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(_item != nil)
@@ -128,15 +147,20 @@ public:
                 [_pb clearContents];
                 [_pb setString:@"" forType:NSPasteboardTypeString];
             }
-        }
         
-        return S_OK;
+            return S_OK;
+        }
     }
     
     virtual HRESULT ObtainFormats(IAvnStringArray** ppv) override
     {
-        *ppv = CreateAvnStringArray(_item == nil ? [_pb types] : [_item types]);
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = CreateAvnStringArray(_item == nil ? [_pb types] : [_item types]);
+            return S_OK;
+        }
     }
 };
 

native/Avalonia.Native/src/OSX/controlhost.mm

@@ -16,11 +16,16 @@ public:
     
     virtual HRESULT CreateDefaultChild(void* parent, void** retOut) override
     {
-        NSView* view = [NSView new];
-        [view setWantsLayer: true];
+        START_COM_CALL;
         
-        *retOut = (__bridge_retained void*)view;
-        return S_OK;
+        @autoreleasepool
+        {
+            NSView* view = [NSView new];
+            [view setWantsLayer: true];
+            
+            *retOut = (__bridge_retained void*)view;
+            return S_OK;
+        }
     };
     
     virtual IAvnNativeControlHostTopLevelAttachment* CreateAttachment() override
@@ -69,32 +74,42 @@ public:
     
     virtual HRESULT InitializeWithChildHandle(void* child) override
     {
-        if(_child != nil)
-            return E_FAIL;
-        _child = (__bridge NSView*)child;
-        if(_child == nil)
-            return E_FAIL;
-        [_holder addSubview:_child];
-        [_child setHidden: false];
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            if(_child != nil)
+                return E_FAIL;
+            _child = (__bridge NSView*)child;
+            if(_child == nil)
+                return E_FAIL;
+            [_holder addSubview:_child];
+            [_child setHidden: false];
+            return S_OK;
+        }
     };
     
     virtual HRESULT AttachTo(IAvnNativeControlHost* host) override
     {
-        if(host == nil)
-        {
-            [_holder removeFromSuperview];
-            [_holder setHidden: true];
-        }
-        else
+        START_COM_CALL;
+        
+        @autoreleasepool
         {
-            AvnNativeControlHost* chost = dynamic_cast<AvnNativeControlHost*>(host);
-            if(chost == nil || chost->View == nil)
-                return E_FAIL;
-            [_holder setHidden:true];
-            [chost->View addSubview:_holder];
+            if(host == nil)
+            {
+                [_holder removeFromSuperview];
+                [_holder setHidden: true];
+            }
+            else
+            {
+                AvnNativeControlHost* chost = dynamic_cast<AvnNativeControlHost*>(host);
+                if(chost == nil || chost->View == nil)
+                    return E_FAIL;
+                [_holder setHidden:true];
+                [chost->View addSubview:_holder];
+            }
+            return S_OK;
         }
-        return S_OK;
     };
     
     virtual void ShowInBounds(float x, float y, float width, float height) override

native/Avalonia.Native/src/OSX/cursor.mm

@@ -53,36 +53,46 @@ public:
     
     virtual HRESULT GetCursor (AvnStandardCursorType cursorType, IAvnCursor** retOut) override
     {
-        *retOut = s_cursorMap[cursorType];
+        START_COM_CALL;
         
-        if(*retOut != nullptr)
+        @autoreleasepool
         {
-            (*retOut)->AddRef();
-        }
+            *retOut = s_cursorMap[cursorType];
             
-        return S_OK;
+            if(*retOut != nullptr)
+            {
+                (*retOut)->AddRef();
+            }
+                
+            return S_OK;
+        }
     }
     
     virtual HRESULT CreateCustomCursor (void* bitmapData, size_t length, AvnPixelSize hotPixel, IAvnCursor** retOut) override
     {
-        if(bitmapData == nullptr || retOut == nullptr)
+        START_COM_CALL;
+        
+        @autoreleasepool
         {
-            return E_POINTER;
+            if(bitmapData == nullptr || retOut == nullptr)
+            {
+                return E_POINTER;
+            }
+            
+            NSData *imageData = [NSData dataWithBytes:bitmapData length:length];
+            NSImage *image = [[NSImage alloc] initWithData:imageData];
+            
+            
+            NSPoint hotSpot;
+            hotSpot.x = hotPixel.Width;
+            hotSpot.y = hotPixel.Height;
+            
+            *retOut = new Cursor([[NSCursor new] initWithImage: image hotSpot: hotSpot]);
+            
+            (*retOut)->AddRef();
+            
+            return S_OK;
         }
-        
-        NSData *imageData = [NSData dataWithBytes:bitmapData length:length];
-        NSImage *image = [[NSImage alloc] initWithData:imageData];
-        
-        
-        NSPoint hotSpot;
-        hotSpot.x = hotPixel.Width;
-        hotSpot.y = hotPixel.Height;
-        
-        *retOut = new Cursor([[NSCursor new] initWithImage: image hotSpot: hotSpot]);
-        
-        (*retOut)->AddRef();
-        
-        return S_OK;
     }
 };
 

native/Avalonia.Native/src/OSX/main.mm

@@ -107,27 +107,42 @@ public:
     
     virtual HRESULT SetApplicationTitle(char* utf8String) override
     {
-        auto appTitle = [NSString stringWithUTF8String: utf8String];
+        START_COM_CALL;
         
-        [[NSProcessInfo processInfo] setProcessName:appTitle];
-        
-        
-        SetProcessName(appTitle);
-        
-        return S_OK;
+        @autoreleasepool
+        {
+            auto appTitle = [NSString stringWithUTF8String: utf8String];
+            
+            [[NSProcessInfo processInfo] setProcessName:appTitle];
+            
+            
+            SetProcessName(appTitle);
+            
+            return S_OK;
+        }
     }
     
     virtual HRESULT SetShowInDock(int show)  override
     {
-        AvnDesiredActivationPolicy = show
-            ? NSApplicationActivationPolicyRegular : NSApplicationActivationPolicyAccessory;
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            AvnDesiredActivationPolicy = show
+                ? NSApplicationActivationPolicyRegular : NSApplicationActivationPolicyAccessory;
+            return S_OK;
+        }
     }
     
     virtual HRESULT SetDisableDefaultApplicationMenuItems (bool enabled) override
     {
-        SetAutoGenerateDefaultAppMenuItems(!enabled);
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            SetAutoGenerateDefaultAppMenuItems(!enabled);
+            return S_OK;
+        }
     }
 };
 
@@ -165,6 +180,8 @@ public:
     FORWARD_IUNKNOWN()
     virtual HRESULT Initialize(IAvnGCHandleDeallocatorCallback* deallocator, IAvnApplicationEvents* events) override
     {
+        START_COM_CALL;
+        
         _deallocator = deallocator;
         @autoreleasepool{
             [[ThreadingInitializer new] do];
@@ -180,89 +197,154 @@ public:
     
     virtual HRESULT CreateWindow(IAvnWindowEvents* cb, IAvnGlContext* gl, IAvnWindow** ppv)  override
     {
-        if(cb == nullptr || ppv == nullptr)
-            return E_POINTER;
-        *ppv = CreateAvnWindow(cb, gl);
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            if(cb == nullptr || ppv == nullptr)
+                return E_POINTER;
+            *ppv = CreateAvnWindow(cb, gl);
+            return S_OK;
+        }
     };
     
     virtual HRESULT CreatePopup(IAvnWindowEvents* cb, IAvnGlContext* gl, IAvnPopup** ppv) override
     {
-        if(cb == nullptr || ppv == nullptr)
-            return E_POINTER;
+        START_COM_CALL;
         
-        *ppv = CreateAvnPopup(cb, gl);
-        return S_OK;
+        @autoreleasepool
+        {
+            if(cb == nullptr || ppv == nullptr)
+                return E_POINTER;
+            
+            *ppv = CreateAvnPopup(cb, gl);
+            return S_OK;
+        }
     }
     
     virtual HRESULT CreatePlatformThreadingInterface(IAvnPlatformThreadingInterface** ppv)  override
     {
-        *ppv = CreatePlatformThreading();
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = CreatePlatformThreading();
+            return S_OK;
+        }
     }
     
     virtual HRESULT CreateSystemDialogs(IAvnSystemDialogs** ppv) override
     {
-        *ppv = ::CreateSystemDialogs();
-        return  S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = ::CreateSystemDialogs();
+            return  S_OK;
+        }
     }
     
     virtual HRESULT CreateScreens (IAvnScreens** ppv) override
     {
-        *ppv = ::CreateScreens ();
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = ::CreateScreens ();
+            return S_OK;
+        }
     }
 
     virtual HRESULT CreateClipboard(IAvnClipboard** ppv) override
     {
-        *ppv = ::CreateClipboard (nil, nil);
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = ::CreateClipboard (nil, nil);
+            return S_OK;
+        }
     }
     
     virtual HRESULT CreateDndClipboard(IAvnClipboard** ppv) override
     {
-        *ppv = ::CreateClipboard (nil, [NSPasteboardItem new]);
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = ::CreateClipboard (nil, [NSPasteboardItem new]);
+            return S_OK;
+        }
     }
 
     virtual HRESULT CreateCursorFactory(IAvnCursorFactory** ppv) override
     {
-        *ppv = ::CreateCursorFactory();
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = ::CreateCursorFactory();
+            return S_OK;
+        }
     }
     
     virtual HRESULT ObtainGlDisplay(IAvnGlDisplay** ppv) override
     {
-        auto rv = ::GetGlDisplay();
-        if(rv == NULL)
-            return E_FAIL;
-        rv->AddRef();
-        *ppv = rv;
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            auto rv = ::GetGlDisplay();
+            if(rv == NULL)
+                return E_FAIL;
+            rv->AddRef();
+            *ppv = rv;
+            return S_OK;
+        }
     }
     
     virtual HRESULT CreateMenu (IAvnMenuEvents* cb, IAvnMenu** ppv) override
     {
-        *ppv = ::CreateAppMenu(cb);
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = ::CreateAppMenu(cb);
+            return S_OK;
+        }
     }
     
     virtual HRESULT CreateMenuItem (IAvnMenuItem** ppv) override
     {
-        *ppv = ::CreateAppMenuItem();
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = ::CreateAppMenuItem();
+            return S_OK;
+        }
     }
     
     virtual HRESULT CreateMenuItemSeparator (IAvnMenuItem** ppv) override
     {
-        *ppv = ::CreateAppMenuItemSeparator();
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            *ppv = ::CreateAppMenuItemSeparator();
+            return S_OK;
+        }
     }
     
     virtual HRESULT SetAppMenu (IAvnMenu* appMenu) override
     {
-        ::SetAppMenu(s_appTitle, appMenu);
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            ::SetAppMenu(s_appTitle, appMenu);
+            return S_OK;
+        }
     }
 };
 

native/Avalonia.Native/src/OSX/menu.mm

@@ -95,6 +95,8 @@ NSMenuItem* AvnAppMenuItem::GetNative()
 
 HRESULT AvnAppMenuItem::SetSubMenu (IAvnMenu* menu)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         if(menu != nullptr)
@@ -114,6 +116,8 @@ HRESULT AvnAppMenuItem::SetSubMenu (IAvnMenu* menu)
 
 HRESULT AvnAppMenuItem::SetTitle (char* utf8String)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         if (utf8String != nullptr)
@@ -128,6 +132,8 @@ HRESULT AvnAppMenuItem::SetTitle (char* utf8String)
 
 HRESULT AvnAppMenuItem::SetGesture (AvnKey key, AvnInputModifiers modifiers)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         if(key != AvnKeyNone)
@@ -183,6 +189,8 @@ HRESULT AvnAppMenuItem::SetGesture (AvnKey key, AvnInputModifiers modifiers)
 
 HRESULT AvnAppMenuItem::SetAction (IAvnPredicateCallback* predicate, IAvnActionCallback* callback)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         _predicate = predicate;
@@ -193,6 +201,8 @@ HRESULT AvnAppMenuItem::SetAction (IAvnPredicateCallback* predicate, IAvnActionC
 
 HRESULT AvnAppMenuItem::SetIsChecked (bool isChecked)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         [_native setState:(isChecked && _isCheckable ? NSOnState : NSOffState)];
@@ -202,6 +212,8 @@ HRESULT AvnAppMenuItem::SetIsChecked (bool isChecked)
 
 HRESULT AvnAppMenuItem::SetToggleType(AvnMenuItemToggleType toggleType)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         switch(toggleType)
@@ -231,6 +243,8 @@ HRESULT AvnAppMenuItem::SetToggleType(AvnMenuItemToggleType toggleType)
 
 HRESULT AvnAppMenuItem::SetIcon(void *data, size_t length)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         if(data != nullptr)
@@ -317,6 +331,8 @@ void AvnAppMenu::RaiseClosed()
 
 HRESULT AvnAppMenu::InsertItem(int index, IAvnMenuItem *item)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         if([_native hasGlobalMenuItem])
@@ -337,6 +353,8 @@ HRESULT AvnAppMenu::InsertItem(int index, IAvnMenuItem *item)
 
 HRESULT AvnAppMenu::RemoveItem (IAvnMenuItem* item)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         auto avnMenuItem = dynamic_cast<AvnAppMenuItem*>(item);
@@ -352,6 +370,8 @@ HRESULT AvnAppMenu::RemoveItem (IAvnMenuItem* item)
 
 HRESULT AvnAppMenu::SetTitle (char* utf8String)
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         if (utf8String != nullptr)
@@ -365,6 +385,8 @@ HRESULT AvnAppMenu::SetTitle (char* utf8String)
 
 HRESULT AvnAppMenu::Clear()
 {
+    START_COM_CALL;
+    
     @autoreleasepool
     {
         [_native removeAllItems];

native/Avalonia.Native/src/OSX/platformthreading.mm

@@ -114,6 +114,8 @@ public:
     
     virtual HRESULT RunLoop(IAvnLoopCancellation* cancel) override
     {
+        START_COM_CALL;
+        
         auto can = dynamic_cast<LoopCancellation*>(cancel);
         if(can->Cancelled)
             return S_OK;

native/Avalonia.Native/src/OSX/rendertarget.mm

@@ -247,6 +247,8 @@ public:
     
     virtual HRESULT GetPixelSize(AvnPixelSize* ret)  override
     {
+        START_COM_CALL;
+        
         if(!_surface)
             return E_FAIL;
         *ret = _surface->size;
@@ -255,6 +257,8 @@ public:
     
     virtual HRESULT GetScaling(double* ret)  override
     {
+        START_COM_CALL;
+        
         if(!_surface)
             return E_FAIL;
         *ret = _surface->scale;
@@ -281,6 +285,8 @@ public:
     
     virtual HRESULT BeginDrawing(IAvnGlSurfaceRenderingSession** ret)  override
     {
+        START_COM_CALL;
+        
         ComPtr<IUnknown> releaseContext;
         @synchronized (_target->lock) {
             if(_target->surface == nil)

native/Avalonia.Native/src/OSX/window.mm

@@ -54,6 +54,8 @@ public:
     
     virtual HRESULT ObtainNSWindowHandle(void** ret) override
     {
+        START_COM_CALL;
+        
         if (ret == nullptr)
         {
             return E_POINTER;
@@ -66,6 +68,8 @@ public:
     
     virtual HRESULT ObtainNSWindowHandleRetained(void** ret) override
     {
+        START_COM_CALL;
+        
         if (ret == nullptr)
         {
             return E_POINTER;
@@ -78,6 +82,8 @@ public:
     
     virtual HRESULT ObtainNSViewHandle(void** ret) override
     {
+        START_COM_CALL;
+        
         if (ret == nullptr)
         {
             return E_POINTER;
@@ -90,6 +96,8 @@ public:
     
     virtual HRESULT ObtainNSViewHandleRetained(void** ret) override
     {
+        START_COM_CALL;
+        
         if (ret == nullptr)
         {
             return E_POINTER;
@@ -107,23 +115,27 @@ public:
     
     virtual HRESULT Show(bool activate) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             SetPosition(lastPositionSet);
             UpdateStyle();
             
             [Window setContentView: StandardContainer];
+            [Window setTitle:_lastTitle];
             
             if(ShouldTakeFocusOnShow() && activate)
             {
+                [Window orderFront: Window];
                 [Window makeKeyAndOrderFront:Window];
+                [Window makeFirstResponder:View];
                 [NSApp activateIgnoringOtherApps:YES];
             }
             else
             {
                 [Window orderFront: Window];
             }
-            [Window setTitle:_lastTitle];
             
             _shown = true;
             
@@ -138,6 +150,8 @@ public:
     
     virtual HRESULT Hide () override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(Window != nullptr)
@@ -152,6 +166,8 @@ public:
     
     virtual HRESULT Activate () override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(Window != nullptr)
@@ -166,6 +182,8 @@ public:
     
     virtual HRESULT SetTopMost (bool value) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             [Window setLevel: value ? NSFloatingWindowLevel : NSNormalWindowLevel];
@@ -176,11 +194,16 @@ public:
     
     virtual HRESULT Close() override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if (Window != nullptr)
             {
-                [Window close];
+                auto window = Window;
+                Window = nullptr;
+                
+                [window close];
             }
             
             return S_OK;
@@ -189,6 +212,8 @@ public:
     
     virtual HRESULT GetClientSize(AvnSize* ret) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(ret == nullptr)
@@ -202,8 +227,25 @@ public:
         }
     }
     
+    virtual HRESULT GetFrameSize(AvnSize* ret) override
+    {
+        @autoreleasepool
+        {
+            if(ret == nullptr)
+                return E_POINTER;
+            
+            auto frame = [Window frame];
+            ret->Width = frame.size.width;
+            ret->Height = frame.size.height;
+            
+            return S_OK;
+        }
+    }
+    
     virtual HRESULT GetScaling (double* ret) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(ret == nullptr)
@@ -222,6 +264,8 @@ public:
     
     virtual HRESULT SetMinMaxSize (AvnSize minSize, AvnSize maxSize) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             [Window setMinSize: ToNSSize(minSize)];
@@ -233,6 +277,8 @@ public:
     
     virtual HRESULT Resize(double x, double y) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             auto maxSize = [Window maxSize];
@@ -272,6 +318,8 @@ public:
     
     virtual HRESULT Invalidate (AvnRect rect) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             [View setNeedsDisplayInRect:[View frame]];
@@ -282,6 +330,8 @@ public:
     
     virtual HRESULT SetMainMenu(IAvnMenu* menu) override
     {
+        START_COM_CALL;
+        
         _mainMenu = menu;
         
         auto nativeMenu = dynamic_cast<AvnAppMenu*>(menu);
@@ -300,6 +350,8 @@ public:
     
     virtual HRESULT BeginMoveDrag () override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             auto lastEvent = [View lastMouseDownEvent];
@@ -317,11 +369,15 @@ public:
     
     virtual HRESULT BeginResizeDrag (AvnWindowEdge edge) override
     {
+        START_COM_CALL;
+        
         return S_OK;
     }
     
     virtual HRESULT GetPosition (AvnPoint* ret) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(ret == nullptr)
@@ -342,6 +398,8 @@ public:
     
     virtual HRESULT SetPosition (AvnPoint point) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             lastPositionSet = point;
@@ -353,6 +411,8 @@ public:
     
     virtual HRESULT PointToClient (AvnPoint point, AvnPoint* ret) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(ret == nullptr)
@@ -371,6 +431,8 @@ public:
     
     virtual HRESULT PointToScreen (AvnPoint point, AvnPoint* ret) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(ret == nullptr)
@@ -388,12 +450,16 @@ public:
     
     virtual HRESULT ThreadSafeSetSwRenderedFrame(AvnFramebuffer* fb, IUnknown* dispose) override
     {
+        START_COM_CALL;
+        
         [View setSwRenderedFrame: fb dispose: dispose];
         return S_OK;
     }
     
     virtual HRESULT SetCursor(IAvnCursor* cursor) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             Cursor* avnCursor = dynamic_cast<Cursor*>(cursor);
@@ -423,6 +489,8 @@ public:
     
     virtual HRESULT CreateGlRenderTarget(IAvnGlSurfaceRenderTarget** ppv) override
     {
+        START_COM_CALL;
+        
         if(View == NULL)
             return E_FAIL;
         *ppv = [renderTarget createSurfaceRenderTarget];
@@ -431,6 +499,8 @@ public:
     
     virtual HRESULT CreateNativeControlHost(IAvnNativeControlHost** retOut) override
     {
+        START_COM_CALL;
+        
         if(View == NULL)
             return E_FAIL;
         *retOut = ::CreateNativeControlHost(View);
@@ -439,6 +509,8 @@ public:
     
     virtual HRESULT SetBlurEnabled (bool enable) override
     {
+        START_COM_CALL;
+        
         [StandardContainer ShowBlur:enable];
         
         return S_OK;
@@ -448,6 +520,8 @@ public:
                                               IAvnClipboard* clipboard, IAvnDndResultCallback* cb,
                                               void* sourceHandle) override
     {
+        START_COM_CALL;
+        
         auto item = TryGetPasteboardItem(clipboard);
         [item setString:@"" forType:GetAvnCustomDataType()];
         if(item == nil)
@@ -513,6 +587,7 @@ private:
     bool _fullScreenActive;
     SystemDecorations _decorations;
     AvnWindowState _lastWindowState;
+    AvnWindowState _actualWindowState;
     bool _inSetWindowState;
     NSRect _preZoomSize;
     bool _transitioningWindowState;
@@ -539,6 +614,7 @@ private:
         _transitioningWindowState = false;
         _inSetWindowState = false;
         _lastWindowState = Normal;
+        _actualWindowState = Normal;
         WindowEvents = events;
         [Window setCanBecomeKeyAndMain];
         [Window disableCursorRects];
@@ -547,6 +623,11 @@ private:
     
     void HideOrShowTrafficLights ()
     {
+        if (Window == nil)
+        {
+            return;
+        }
+        
         for (id subview in Window.contentView.superview.subviews) {
             if ([subview isKindOfClass:NSClassFromString(@"NSTitlebarContainerView")]) {
                 NSView *titlebarView = [subview subviews][0];
@@ -573,8 +654,10 @@ private:
     
     virtual HRESULT Show (bool activate) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
-        {            
+        {
             WindowBaseImpl::Show(activate);
             
             HideOrShowTrafficLights();
@@ -585,6 +668,8 @@ private:
     
     virtual HRESULT SetEnabled (bool enable) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             [Window setEnabled:enable];
@@ -594,6 +679,8 @@ private:
     
     virtual HRESULT SetParent (IAvnWindow* parent) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(parent == nullptr)
@@ -633,7 +720,7 @@ private:
     
     void WindowStateChanged () override
     {
-        if(!_inSetWindowState && !_transitioningWindowState)
+        if(_shown && !_inSetWindowState && !_transitioningWindowState)
         {
             AvnWindowState state;
             GetWindowState(&state);
@@ -705,6 +792,8 @@ private:
     
     virtual HRESULT SetCanResize(bool value) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             _canResize = value;
@@ -715,6 +804,8 @@ private:
     
     virtual HRESULT SetDecorations(SystemDecorations value) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             auto currentWindowState = _lastWindowState;
@@ -780,6 +871,8 @@ private:
     
     virtual HRESULT SetTitle (char* utf8title) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             _lastTitle = [NSString stringWithUTF8String:(const char*)utf8title];
@@ -791,6 +884,8 @@ private:
     
     virtual HRESULT SetTitleBarColor(AvnColor color) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             float a = (float)color.Alpha / 255.0f;
@@ -820,6 +915,8 @@ private:
     
     virtual HRESULT GetWindowState (AvnWindowState*ret) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
             if(ret == nullptr)
@@ -853,86 +950,111 @@ private:
     
     virtual HRESULT TakeFocusFromChildren () override
     {
-        if(Window == nil)
-            return S_OK;
-        if([Window isKeyWindow])
-            [Window makeFirstResponder: View];
+        START_COM_CALL;
         
-        return S_OK;
+        @autoreleasepool
+        {
+            if(Window == nil)
+                return S_OK;
+            if([Window isKeyWindow])
+                [Window makeFirstResponder: View];
+            
+            return S_OK;
+        }
     }
     
     virtual HRESULT SetExtendClientArea (bool enable) override
     {
-        _isClientAreaExtended = enable;
+        START_COM_CALL;
         
-        if(enable)
+        @autoreleasepool
         {
-            Window.titleVisibility = NSWindowTitleHidden;
+            _isClientAreaExtended = enable;
             
-            [Window setTitlebarAppearsTransparent:true];
-            
-            auto wantsTitleBar = (_extendClientHints & AvnSystemChrome) || (_extendClientHints & AvnPreferSystemChrome);
-            
-            if (wantsTitleBar)
-            {
-                [StandardContainer ShowTitleBar:true];
-            }
-            else
+            if(enable)
             {
-                [StandardContainer ShowTitleBar:false];
-            }
-            
-            if(_extendClientHints & AvnOSXThickTitleBar)
-            {
-                Window.toolbar = [NSToolbar new];
-                Window.toolbar.showsBaselineSeparator = false;
+                Window.titleVisibility = NSWindowTitleHidden;
+                
+                [Window setTitlebarAppearsTransparent:true];
+                
+                auto wantsTitleBar = (_extendClientHints & AvnSystemChrome) || (_extendClientHints & AvnPreferSystemChrome);
+                
+                if (wantsTitleBar)
+                {
+                    [StandardContainer ShowTitleBar:true];
+                }
+                else
+                {
+                    [StandardContainer ShowTitleBar:false];
+                }
+                
+                if(_extendClientHints & AvnOSXThickTitleBar)
+                {
+                    Window.toolbar = [NSToolbar new];
+                    Window.toolbar.showsBaselineSeparator = false;
+                }
+                else
+                {
+                    Window.toolbar = nullptr;
+                }
             }
             else
             {
+                Window.titleVisibility = NSWindowTitleVisible;
                 Window.toolbar = nullptr;
+                [Window setTitlebarAppearsTransparent:false];
+                View.layer.zPosition = 0;
             }
+            
+            [Window setIsExtended:enable];
+            
+            HideOrShowTrafficLights();
+            
+            UpdateStyle();
+            
+            return S_OK;
         }
-        else
-        {
-            Window.titleVisibility = NSWindowTitleVisible;
-            Window.toolbar = nullptr;
-            [Window setTitlebarAppearsTransparent:false];
-            View.layer.zPosition = 0;
-        }
-        
-        [Window setIsExtended:enable];
-        
-        HideOrShowTrafficLights();
-        
-        UpdateStyle();
-        
-        return S_OK;
     }
     
     virtual HRESULT SetExtendClientAreaHints (AvnExtendClientAreaChromeHints hints) override
     {
-        _extendClientHints = hints;
+        START_COM_CALL;
         
-        SetExtendClientArea(_isClientAreaExtended);
-        return S_OK;
+        @autoreleasepool
+        {
+            _extendClientHints = hints;
+            
+            SetExtendClientArea(_isClientAreaExtended);
+            return S_OK;
+        }
     }
     
     virtual HRESULT GetExtendTitleBarHeight (double*ret) override
     {
-        if(ret == nullptr)
+        START_COM_CALL;
+        
+        @autoreleasepool
         {
-            return E_POINTER;
+            if(ret == nullptr)
+            {
+                return E_POINTER;
+            }
+            
+            *ret = [Window getExtendedTitleBarHeight];
+            
+            return S_OK;
         }
-        
-        *ret = [Window getExtendedTitleBarHeight];
-        
-        return S_OK;
     }
     
     virtual HRESULT SetExtendTitleBarHeight (double value) override
     {
-        [StandardContainer SetTitleBarHeightHint:value];
-        return S_OK;
+        START_COM_CALL;
+        
+        @autoreleasepool
+        {
+            [StandardContainer SetTitleBarHeightHint:value];
+            return S_OK;
+        }
     }
     
     void EnterFullScreenMode ()
@@ -961,16 +1083,23 @@ private:
     
     virtual HRESULT SetWindowState (AvnWindowState state) override
     {
+        START_COM_CALL;
+        
         @autoreleasepool
         {
-            if(_lastWindowState == state)
+            if(Window == nullptr)
+            {
+                return  S_OK;
+            }
+            
+            if(_actualWindowState == state)
             {
                 return S_OK;
             }
             
             _inSetWindowState = true;
             
-            auto currentState = _lastWindowState;
+            auto currentState = _actualWindowState;
             _lastWindowState = state;
             
             if(currentState == Normal)
@@ -1049,8 +1178,11 @@ private:
                         }
                         break;
                 }
+                
+                _actualWindowState = _lastWindowState;
             }
             
+            
             _inSetWindowState = false;
             
             return S_OK;
@@ -1906,7 +2038,7 @@ NSArray* AllLoopModes = [NSArray arrayWithObjects: NSDefaultRunLoopMode, NSEvent
     {
         if(![self windowShouldClose:self]) return;
     }
-    
+
     [self close];
 }
 

samples/ControlCatalog/MainWindow.xaml.cs

@@ -17,7 +17,10 @@ namespace ControlCatalog
         public MainWindow()
         {
             this.InitializeComponent();
-            this.AttachDevTools();
+            this.AttachDevTools(new Avalonia.Diagnostics.DevToolsOptions()
+            {
+                StartupScreenIndex = 1,
+            });
             //Renderer.DrawFps = true;
             //Renderer.DrawDirtyRects = Renderer.DrawFps = true;
 

samples/ControlCatalog/Pages/ContextFlyoutPage.axaml

@@ -84,13 +84,13 @@
                         <Button Name="CutButton" Command="{Binding $parent[TextBox].Cut}" IsEnabled="{Binding $parent[TextBox].CanCut}">
                             <PathIcon Width="14" Height="14" Data="M5.22774,2.08072 C5.43359778,1.94704 5.7011484,1.98419259 5.86368634,2.15675215 L5.91939,2.22774 L12.5191,12.3904 C12.956,12.1419 13.4614,12.0000019 14,12.0000019 C15.6569,12.0000019 17,13.3431 17,15.0000019 C17,16.6569 15.6569,18.0000019 14,18.0000019 C12.3431,18.0000019 11,16.6569 11,15.0000019 C11,14.3201402 11.226152,13.693011 11.6073785,13.1899092 L11.7401,13.0269 L10,10.3474 L8.25991,13.0269 C8.72078,13.5543 9,14.2446 9,15.0000019 C9,16.6569 7.65685,18.0000019 6,18.0000019 C4.34315,18.0000019 3,16.6569 3,15.0000019 C3,13.3431 4.34315,12.0000019 6,12.0000019 C6.46163143,12.0000019 6.89890041,12.1042536 7.28955831,12.2905296 L7.4809,12.3904 L9.40382,9.42936 L5.08072,2.77238 C4.93033,2.54079 4.99615,2.23112 5.22774,2.08072 Z M14,13 C12.8954,13 12,13.8954 12,15 C12,16.1046 12.8954,17 14,17 C15.1046,17 16,16.1046 16,15 C16,13.8954 15.1046,13 14,13 Z M6,13 C4.89543,13 4,13.8954 4,15 C4,16.1046 4.89543,17 6,17 C7.10457,17 8,16.1046 8,15 C8,13.8954 7.10457,13 6,13 Z M14.7723,2.08072 C15.0039,2.23112 15.0697,2.54079 14.9193,2.77238 L11.1924,8.51133 L10.5962,7.59329 L14.0806,2.22774 C14.231,1.99615 14.5407,1.93033 14.7723,2.08072 Z" />
                         </Button>
-                        <Button Name="CopyButton" Content="Copy" Command="{Binding $parent[TextBox].Copy}" IsEnabled="{Binding $parent[TextBox].CanCopy}">
+                        <Button Name="CopyButton" Command="{Binding $parent[TextBox].Copy}" IsEnabled="{Binding $parent[TextBox].CanCopy}">
                             <PathIcon Width="14" Height="14" Data="M5.50280381,4.62704038 L5.5,6.75 L5.5,17.2542087 C5.5,19.0491342 6.95507456,20.5042087 8.75,20.5042087 L17.3662868,20.5044622 C17.057338,21.3782241 16.2239751,22.0042087 15.2444057,22.0042087 L8.75,22.0042087 C6.12664744,22.0042087 4,19.8775613 4,17.2542087 L4,6.75 C4,5.76928848 4.62744523,4.93512464 5.50280381,4.62704038 Z M17.75,2 C18.9926407,2 20,3.00735931 20,4.25 L20,17.25 C20,18.4926407 18.9926407,19.5 17.75,19.5 L8.75,19.5 C7.50735931,19.5 6.5,18.4926407 6.5,17.25 L6.5,4.25 C6.5,3.00735931 7.50735931,2 8.75,2 L17.75,2 Z M17.75,3.5 L8.75,3.5 C8.33578644,3.5 8,3.83578644 8,4.25 L8,17.25 C8,17.6642136 8.33578644,18 8.75,18 L17.75,18 C18.1642136,18 18.5,17.6642136 18.5,17.25 L18.5,4.25 C18.5,3.83578644 18.1642136,3.5 17.75,3.5 Z" />
                         </Button>
-                        <Button Name="PasteButton" Content="Paste" Command="{Binding $parent[TextBox].Paste}" IsEnabled="{Binding $parent[TextBox].CanPaste}">
+                        <Button Name="PasteButton" Command="{Binding $parent[TextBox].Paste}" IsEnabled="{Binding $parent[TextBox].CanPaste}">
                             <PathIcon Width="14" Height="14" Data="M13.75,2 C14.940864,2 15.9156449,2.92516159 15.9948092,4.09595119 L16,4.25 L16,4.25 C16,4.16530567 15.9953205,4.0817043 15.9862059,3.99944035 L17.75,4 C18.9926407,4 20,5.00735931 20,6.25 L20,19.75 C20,20.9926407 18.9926407,22 17.75,22 L6.25,22 C5.00735931,22 4,20.9926407 4,19.75 L4,6.25 C4,5.00735931 5.00735931,4 6.25,4 L8.01379413,3.99944035 C8.00733496,4.05773764 8.00310309,4.11670658 8.00118552,4.17626017 L8,4.25 C8,3.00735931 9.00735931,2 10.25,2 L13.75,2 Z M13.75,6.5 L10.25,6.5 C9.45594921,6.5 8.75796956,6.08867052 8.357512,5.4674625 L8.37902077,5.50019943 L8.37902077,5.50019943 L6.25,5.5 C5.83578644,5.5 5.5,5.83578644 5.5,6.25 L5.5,19.75 C5.5,20.1642136 5.83578644,20.5 6.25,20.5 L17.75,20.5 C18.1642136,20.5 18.5,20.1642136 18.5,19.75 L18.5,6.25 C18.5,5.83578644 18.1642136,5.5 17.75,5.5 L15.6209792,5.50019943 L15.642488,5.4674625 C15.2420304,6.08867052 14.5440508,6.5 13.75,6.5 Z M13.75,3.5 L10.25,3.5 C9.83578644,3.5 9.5,3.83578644 9.5,4.25 C9.5,4.66421356 9.83578644,5 10.25,5 L13.75,5 C14.1642136,5 14.5,4.66421356 14.5,4.25 C14.5,3.83578644 14.1642136,3.5 13.75,3.5 Z" />
                         </Button>
-                        <Button Name="ClearButton" Content="Clear" Command="{Binding $parent[TextBox].Clear}">
+                        <Button Name="ClearButton" Command="{Binding $parent[TextBox].Clear}">
                             <PathIcon Width="14" Height="14" Data="M3.52499419,3.71761187 L3.61611652,3.61611652 C4.0717282,3.16050485 4.79154862,3.13013074 5.28238813,3.52499419 L5.38388348,3.61611652 L14,12.233 L22.6161165,3.61611652 C23.1042719,3.12796116 23.8957281,3.12796116 24.3838835,3.61611652 C24.8720388,4.10427189 24.8720388,4.89572811 24.3838835,5.38388348 L15.767,14 L24.3838835,22.6161165 C24.8394952,23.0717282 24.8698693,23.7915486 24.4750058,24.2823881 L24.3838835,24.3838835 C23.9282718,24.8394952 23.2084514,24.8698693 22.7176119,24.4750058 L22.6161165,24.3838835 L14,15.767 L5.38388348,24.3838835 C4.89572811,24.8720388 4.10427189,24.8720388 3.61611652,24.3838835 C3.12796116,23.8957281 3.12796116,23.1042719 3.61611652,22.6161165 L12.233,14 L3.61611652,5.38388348 C3.16050485,4.9282718 3.13013074,4.20845138 3.52499419,3.71761187 L3.61611652,3.61611652 L3.52499419,3.71761187 Z" />
                         </Button>
                     </StackPanel>

samples/RenderDemo/MainWindow.xaml

@@ -36,6 +36,9 @@
       <TabItem Header="Transitions">
         <pages:TransitionsPage/>
       </TabItem>
+      <TabItem Header="Custom Animator">
+        <pages:CustomAnimatorPage/>
+      </TabItem>
       <TabItem Header="Clipping">
         <pages:ClippingPage/>
       </TabItem>

samples/RenderDemo/Pages/CustomAnimatorPage.xaml

@@ -0,0 +1,25 @@
+<UserControl 
+  xmlns="https://github.com/avaloniaui" 
+  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
+  xmlns:pages="clr-namespace:RenderDemo.Pages"
+  x:Class="RenderDemo.Pages.CustomAnimatorPage"
+  MaxWidth="600">
+  <Grid>
+    <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center">
+      <TextBlock.Styles>
+        <Style Selector="TextBlock">
+          <Style.Animations>
+            <Animation Duration="0:0:1" IterationCount="Infinite">
+              <KeyFrame Cue="0%">
+                <Setter Property="Text" Value="" Animation.Animator="{x:Type pages:CustomStringAnimator}"/>
+              </KeyFrame>
+              <KeyFrame Cue="100%">
+                <Setter Property="Text" Value="0123456789" Animation.Animator="{x:Type pages:CustomStringAnimator}"/>
+              </KeyFrame>
+            </Animation>
+          </Style.Animations>
+        </Style>
+      </TextBlock.Styles>
+    </TextBlock>
+  </Grid>
+</UserControl>

samples/RenderDemo/Pages/CustomAnimatorPage.xaml.cs

@@ -0,0 +1,27 @@
+using System.Reactive.Linq;
+using Avalonia;
+using Avalonia.Animation;
+using Avalonia.Controls;
+using Avalonia.Controls.Shapes;
+using Avalonia.Data;
+using Avalonia.Input;
+using Avalonia.Interactivity;
+using Avalonia.Markup.Xaml;
+using Avalonia.Media;
+using RenderDemo.ViewModels;
+
+namespace RenderDemo.Pages
+{
+    public class CustomAnimatorPage : UserControl
+    {
+        public CustomAnimatorPage()
+        {
+            InitializeComponent();
+        }
+
+        private void InitializeComponent()
+        {
+            AvaloniaXamlLoader.Load(this);
+        }
+    }
+}

samples/RenderDemo/Pages/CustomStringAnimator.cs

@@ -0,0 +1,16 @@
+using Avalonia.Animation.Animators;
+
+namespace RenderDemo.Pages
+{
+    public class CustomStringAnimator : Animator<string>
+    {
+        public override string Interpolate(double progress, string oldValue, string newValue)
+        {
+            if (newValue.Length == 0) return "";
+            var step = 1.0 / newValue.Length;
+            var length = (int)(progress / step);
+            var result = newValue.Substring(0, length + 1);
+            return result;
+        }
+    }
+}

samples/RenderDemo/Pages/TransitionsPage.xaml

@@ -141,6 +141,39 @@
       <Style Selector="Border.Shadow:pointerover">
         <Setter Property="BoxShadow" Value="inset 30 30 20 30 Green, 20 40 20 10 Red"/>
       </Style>
+
+      <Style Selector="Border.Rect10">
+        <Setter Property="Transitions">
+          <Transitions>
+            <BrushTransition Property="Background" Duration="0:0:0.5" />
+          </Transitions>
+        </Setter>
+        <Setter Property="Background" Value="Red" />
+      </Style>
+
+      <Style Selector="Border.Rect10:pointerover">
+        <Setter Property="Background" Value="Orange" />
+      </Style>
+
+      <Style Selector="Border.Rect11">
+        <Setter Property="Transitions">
+          <Transitions>
+            <BrushTransition Property="Background" Duration="0:0:0.5" />
+          </Transitions>
+        </Setter>
+        <Setter Property="Background" Value="Red" />
+      </Style>
+
+      <Style Selector="Border.Rect11:pointerover">
+        <Setter Property="Background" >
+          <LinearGradientBrush StartPoint="0%,0%" EndPoint="100%,100%">
+            <LinearGradientBrush.GradientStops>
+              <GradientStop Offset="0" Color="Red"/>
+              <GradientStop Offset="1" Color="Blue"/>
+            </LinearGradientBrush.GradientStops>
+          </LinearGradientBrush>
+        </Setter>
+      </Style>
     </Styles>
   </UserControl.Styles>
 
@@ -166,6 +199,9 @@
 
         <Border Classes="Test Shadow" CornerRadius="10" Child="{x:Null}" />
         <Border Classes="Test Shadow" CornerRadius="0 30 60 0" Child="{x:Null}" />
+
+        <Border Classes="Test Rect10" />
+        <Border Classes="Test Rect11" />
       </WrapPanel>
     </StackPanel>
   </Grid>

samples/Sandbox/Program.cs

@@ -4,12 +4,12 @@ namespace Sandbox
 {
     public class Program
     {
-        static void Main(string[] args)
-        {
+        static void Main(string[] args) => BuildAvaloniaApp()
+            .StartWithClassicDesktopLifetime(args);
+
+        public static AppBuilder BuildAvaloniaApp() =>
             AppBuilder.Configure<App>()
                 .UsePlatformDetect()
-                .LogToTrace()
-                .StartWithClassicDesktopLifetime(args);
-        }
+                .LogToTrace();
     }
 }

src/Android/Avalonia.Android/Platform/SkiaPlatform/TopLevelImpl.cs

@@ -55,6 +55,8 @@ namespace Avalonia.Android.Platform.SkiaPlatform
 
         public virtual Size ClientSize => Size.ToSize(RenderScaling);
 
+        public Size? FrameSize => null;
+
         public IMouseDevice MouseDevice { get; } = new MouseDevice();
 
         public Action Closed { get; set; }

src/Avalonia.Animation/Animation.cs

@@ -3,10 +3,11 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Reactive.Disposables;
 using System.Reactive.Linq;
+using System.Threading;
 using System.Threading.Tasks;
+
 using Avalonia.Animation.Animators;
 using Avalonia.Animation.Easings;
-using Avalonia.Collections;
 using Avalonia.Data;
 using Avalonia.Metadata;
 
@@ -194,6 +195,33 @@ namespace Avalonia.Animation
         [Content]
         public KeyFrames Children { get; } = new KeyFrames();
 
+        // Store values for the Animator attached properties for IAnimationSetter objects.
+        private static readonly Dictionary<IAnimationSetter, Type> s_animators = new Dictionary<IAnimationSetter, Type>();
+
+        /// <summary>
+        /// Gets the value of the Animator attached property for a setter.
+        /// </summary>
+        /// <param name="setter">The animation setter.</param>
+        /// <returns>The property animator type.</returns>
+        public static Type GetAnimator(IAnimationSetter setter)
+        {
+            if (s_animators.TryGetValue(setter, out var type))
+            {
+                return type;
+            }
+            return null;
+        }
+
+        /// <summary>
+        /// Sets the value of the Animator attached property for a setter.
+        /// </summary>
+        /// <param name="setter">The animation setter.</param>
+        /// <param name="value">The property animator value.</param>
+        public static void SetAnimator(IAnimationSetter setter, Type value)
+        {
+            s_animators[setter] = value;
+        }
+
         private readonly static List<(Func<AvaloniaProperty, bool> Condition, Type Animator)> Animators = new List<(Func<AvaloniaProperty, bool>, Type)>
         {
             ( prop => typeof(bool).IsAssignableFrom(prop.PropertyType), typeof(BoolAnimator) ),
@@ -248,7 +276,7 @@ namespace Avalonia.Animation
             {
                 foreach (var setter in keyframe.Setters)
                 {
-                    var handler = GetAnimatorType(setter.Property);
+                    var handler = Animation.GetAnimator(setter) ?? GetAnimatorType(setter.Property);
 
                     if (handler == null)
                     {
@@ -292,7 +320,7 @@ namespace Avalonia.Animation
             return (newAnimatorInstances, subscriptions);
         }
 
-        /// <inheritdocs/>
+        /// <inheritdoc/>
         public IDisposable Apply(Animatable control, IClock clock, IObservable<bool> match, Action onComplete)
         {
             var (animators, subscriptions) = InterpretKeyframes(control);
@@ -317,25 +345,40 @@ namespace Avalonia.Animation
 
                 if (onComplete != null)
                 {
-                    Task.WhenAll(completionTasks).ContinueWith(_ => onComplete());
+                    Task.WhenAll(completionTasks).ContinueWith(
+                        (_, state) => ((Action)state).Invoke(),
+                        onComplete);
                 }
             }
             return new CompositeDisposable(subscriptions);
         }
 
-        /// <inheritdocs/>
-        public Task RunAsync(Animatable control, IClock clock = null)
+        /// <inheritdoc/>
+        public Task RunAsync(Animatable control, IClock clock = null, CancellationToken cancellationToken = default)
         {
+            if (cancellationToken.IsCancellationRequested)
+            {
+                return Task.CompletedTask;
+            }
+
             var run = new TaskCompletionSource<object>();
 
             if (this.IterationCount == IterationCount.Infinite)
                 run.SetException(new InvalidOperationException("Looping animations must not use the Run method."));
 
-            IDisposable subscriptions = null;
+            IDisposable subscriptions = null, cancellation = null;
             subscriptions = this.Apply(control, clock, Observable.Return(true), () =>
             {
-                run.SetResult(null);
+                run.TrySetResult(null);
+                subscriptions?.Dispose();
+                cancellation?.Dispose();
+            });
+
+            cancellation = cancellationToken.Register(() =>
+            {
+                run.TrySetResult(null);
                 subscriptions?.Dispose();
+                cancellation?.Dispose();
             });
 
             return run.Task;

src/Avalonia.Animation/AnimationInstance`1.cs

@@ -5,6 +5,7 @@ using Avalonia.Animation.Animators;
 using Avalonia.Animation.Utils;
 using Avalonia.Data;
 using Avalonia.Reactive;
+using JetBrains.Annotations;
 
 namespace Avalonia.Animation
 {
@@ -36,6 +37,7 @@ namespace Avalonia.Animation
         private IDisposable _timerSub;
         private readonly IClock _baseClock;
         private IClock _clock;
+        private EventHandler<AvaloniaPropertyChangedEventArgs> _propertyChangedDelegate;
 
         public AnimationInstance(Animation animation, Animatable control, Animator<T> animator, IClock baseClock, Action OnComplete, Func<double, T, T> Interpolator)
         {
@@ -45,8 +47,6 @@ namespace Avalonia.Animation
             _onCompleteAction = OnComplete;
             _interpolator = Interpolator;
             _baseClock = baseClock;
-            _neutralValue = (T)_targetControl.GetValue(_animator.Property);
-
             FetchProperties();
         }
 
@@ -80,6 +80,7 @@ namespace Avalonia.Animation
             // Animation may have been stopped before it has finished.
             ApplyFinalFill();
 
+            _targetControl.PropertyChanged -= _propertyChangedDelegate;
             _timerSub?.Dispose();
             _clock.PlayState = PlayState.Stop;
         }
@@ -88,6 +89,9 @@ namespace Avalonia.Animation
         {
             _clock = new Clock(_baseClock);
             _timerSub = _clock.Subscribe(Step);
+            _propertyChangedDelegate ??= ControlPropertyChanged;
+            _targetControl.PropertyChanged += _propertyChangedDelegate;
+            UpdateNeutralValue();
         }
 
         public void Step(TimeSpan frameTick)
@@ -216,5 +220,22 @@ namespace Avalonia.Animation
                 }
             }
         }
+
+        private void UpdateNeutralValue()
+        {
+            var property = _animator.Property;
+            var baseValue = _targetControl.GetBaseValue(property, BindingPriority.LocalValue);
+
+            _neutralValue = baseValue != AvaloniaProperty.UnsetValue ?
+                (T)baseValue : (T)_targetControl.GetValue(property);
+        }
+
+        private void ControlPropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        {
+            if (e.Property == _animator.Property && e.Priority > BindingPriority.Animation)
+            {
+                UpdateNeutralValue();
+            }
+        }
     }
 }

src/Avalonia.Animation/ApiCompatBaseline.txt

@@ -0,0 +1,6 @@
+Compat issues with assembly Avalonia.Animation:
+MembersMustExist : Member 'public System.Threading.Tasks.Task Avalonia.Animation.Animation.RunAsync(Avalonia.Animation.Animatable, Avalonia.Animation.IClock)' does not exist in the implementation but it does exist in the contract.
+InterfacesShouldHaveSameMembers : Interface member 'public System.Threading.Tasks.Task Avalonia.Animation.IAnimation.RunAsync(Avalonia.Animation.Animatable, Avalonia.Animation.IClock)' is present in the contract but not in the implementation.
+MembersMustExist : Member 'public System.Threading.Tasks.Task Avalonia.Animation.IAnimation.RunAsync(Avalonia.Animation.Animatable, Avalonia.Animation.IClock)' does not exist in the implementation but it does exist in the contract.
+InterfacesShouldHaveSameMembers : Interface member 'public System.Threading.Tasks.Task Avalonia.Animation.IAnimation.RunAsync(Avalonia.Animation.Animatable, Avalonia.Animation.IClock, System.Threading.CancellationToken)' is present in the implementation but not in the contract.
+Total Issues: 4

src/Avalonia.Animation/IAnimation.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace Avalonia.Animation
@@ -16,6 +17,6 @@ namespace Avalonia.Animation
         /// <summary>
         /// Run the animation on the specified control.
         /// </summary>
-        Task RunAsync(Animatable control, IClock clock);
+        Task RunAsync(Animatable control, IClock clock, CancellationToken cancellationToken = default);
     }
 }

src/Avalonia.Base/AvaloniaObjectExtensions.cs

@@ -585,6 +585,30 @@ namespace Avalonia
             });
         }
 
+        /// <summary>
+        /// Subscribes to a property changed notifications for changes that originate from a
+        /// <typeparamref name="TTarget"/>.
+        /// </summary>
+        /// <typeparam name="TTarget">The type of the property change sender.</typeparam>
+        /// /// <typeparam name="TValue">The type of the property..</typeparam>
+        /// <param name="observable">The property changed observable.</param>
+        /// <param name="action">
+        /// The method to call. The parameters are the sender and the event args.
+        /// </param>
+        /// <returns>A disposable that can be used to terminate the subscription.</returns>
+        public static IDisposable AddClassHandler<TTarget, TValue>(
+            this IObservable<AvaloniaPropertyChangedEventArgs<TValue>> observable,
+            Action<TTarget, AvaloniaPropertyChangedEventArgs<TValue>> action) where TTarget : AvaloniaObject
+        {
+            return observable.Subscribe(e =>
+            {
+                if (e.Sender is TTarget target)
+                {
+                    action(target, e);
+                }
+            });
+        }
+
         /// <summary>
         /// Subscribes to a property changed notifications for changes that originate from a
         /// <typeparamref name="TTarget"/>.

src/Avalonia.Base/Metadata/NullableAttributes.cs

@@ -1,6 +1,5 @@
 #pragma warning disable MA0048 // File name must match type name
 #define INTERNAL_NULLABLE_ATTRIBUTES
-#if NETSTANDARD2_0 ||  NETCOREAPP2_0 ||  NETCOREAPP2_1 ||  NETCOREAPP2_2 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48
 
 // https://github.com/dotnet/corefx/blob/48363ac826ccf66fbe31a5dcb1dc2aab9a7dd768/src/Common/src/CoreLib/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
 
@@ -10,6 +9,7 @@
 
 namespace System.Diagnostics.CodeAnalysis
 {
+#if NETSTANDARD2_0 || NETCOREAPP2_0 || NETCOREAPP2_1 || NETCOREAPP2_2 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48
     /// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
     [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
 #if INTERNAL_NULLABLE_ATTRIBUTES
@@ -136,5 +136,82 @@ namespace System.Diagnostics.CodeAnalysis
         /// <summary>Gets the condition parameter value.</summary>
         public bool ParameterValue { get; }
     }
-}
+#endif // NETSTANDARD2_0 attributes
+
+#if NETSTANDARD2_1 || NETSTANDARD2_0 || NETCOREAPP2_0 || NETCOREAPP2_1 || NETCOREAPP2_2 || NETCOREAPP3_1 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48
+    /// <summary>
+    /// Specifies that the method or property will ensure that the listed field and property members have
+    /// not-<see langword="null"/> values.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
 #endif
+    sealed class MemberNotNullAttribute : Attribute
+    {
+        /// <summary>Gets field or property member names.</summary>
+        public string[] Members { get; }
+
+        /// <summary>Initializes the attribute with a field or property member.</summary>
+        /// <param name="member">The field or property member that is promised to be not-null.</param>
+        public MemberNotNullAttribute(string member)
+        {
+            Members = new[] { member };
+        }
+
+        /// <summary>Initializes the attribute with the list of field and property members.</summary>
+        /// <param name="members">The list of field and property members that are promised to be not-null.</param>
+        public MemberNotNullAttribute(params string[] members)
+        {
+            Members = members;
+        }
+    }
+
+    /// <summary>
+    /// Specifies that the method or property will ensure that the listed field and property members have
+    /// non-<see langword="null"/> values when returning with the specified return value condition.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+    sealed class MemberNotNullWhenAttribute : Attribute
+    {
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+
+        /// <summary>Gets field or property member names.</summary>
+        public string[] Members { get; }
+
+        /// <summary>Initializes the attribute with the specified return value condition and a field or property member.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value,
+        /// the associated parameter will not be <see langword="null"/>.
+        /// </param>
+        /// <param name="member">The field or property member that is promised to be not-<see langword="null"/>.</param>
+        public MemberNotNullWhenAttribute(bool returnValue, string member)
+        {
+            ReturnValue = returnValue;
+            Members = new[] { member };
+        }
+
+        /// <summary>Initializes the attribute with the specified return value condition and list of field and property members.
+        /// </summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value,
+        /// the associated parameter will not be <see langword="null"/>.
+        /// </param>
+        /// <param name="members">The list of field and property members that are promised to be not-null.</param>
+        public MemberNotNullWhenAttribute(bool returnValue, params string[] members)
+        {
+            ReturnValue = returnValue;
+            Members = members;
+        }
+    }
+#endif // NETSTANDARD2_1 attributes
+}
+

src/Avalonia.Controls.DataGrid/DataGrid.cs

@@ -75,7 +75,6 @@ namespace Avalonia.Controls
         private const double DATAGRID_defaultMinColumnWidth = 20;
         private const double DATAGRID_defaultMaxColumnWidth = double.PositiveInfinity;
 
-        private List<Exception> _validationErrors;
         private List<Exception> _bindingValidationErrors;
         private IDisposable _validationSubscription;
 
@@ -102,7 +101,6 @@ namespace Avalonia.Controls
         private bool _areHandlersSuspended;
         private bool _autoSizingColumns;
         private IndexToValueTable<bool> _collapsedSlotsTable;
-        private DataGridCellCoordinates _currentCellCoordinates;
         private Control _clickedElement;
 
         // used to store the current column during a Reset
@@ -141,7 +139,6 @@ namespace Avalonia.Controls
         private DataGridSelectedItemsCollection _selectedItems;
         private bool _temporarilyResetCurrentCell;
         private object _uneditedValue; // Represents the original current cell value at the time it enters editing mode.
-        private ICellEditBinding _currentCellEditBinding;
 
         // An approximation of the sum of the heights in pixels of the scrolling rows preceding
         // the first displayed scrolling row.  Since the scrolled off rows are discarded, the grid

src/Avalonia.Controls.DataGrid/DataGridRows.cs

@@ -425,7 +425,7 @@ namespace Avalonia.Controls
                 UpdateDisplayedRows(DisplayData.FirstScrollingSlot, CellsHeight);
             }
 
-            if (DisplayData.FirstScrollingSlot < slot && DisplayData.LastScrollingSlot > slot)
+            if (DisplayData.FirstScrollingSlot < slot && (DisplayData.LastScrollingSlot > slot || DisplayData.LastScrollingSlot == -1))
             {
                 // The row is already displayed in its entirety
                 return true;

src/Avalonia.Controls/ApiCompatBaseline.txt

@@ -4,30 +4,7 @@ InterfacesShouldHaveSameMembers : Interface member 'public System.Boolean Avalon
 InterfacesShouldHaveSameMembers : Interface member 'public void Avalonia.Controls.IMenuItem.StaysOpenOnClick.set(System.Boolean)' is present in the implementation but not in the contract.
 InterfacesShouldHaveSameMembers : Interface member 'public void Avalonia.Controls.INativeMenuExporterEventsImplBridge.RaiseClosed()' is present in the implementation but not in the contract.
 InterfacesShouldHaveSameMembers : Interface member 'public void Avalonia.Controls.INativeMenuExporterEventsImplBridge.RaiseOpening()' is present in the implementation but not in the contract.
-MembersMustExist : Member 'public Avalonia.DirectProperty<Avalonia.Controls.NumericUpDown, System.Double> Avalonia.DirectProperty<Avalonia.Controls.NumericUpDown, System.Double> Avalonia.Controls.NumericUpDown.ValueProperty' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public Avalonia.StyledProperty<System.Double> Avalonia.StyledProperty<System.Double> Avalonia.Controls.NumericUpDown.IncrementProperty' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public Avalonia.StyledProperty<System.Double> Avalonia.StyledProperty<System.Double> Avalonia.Controls.NumericUpDown.MaximumProperty' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public Avalonia.StyledProperty<System.Double> Avalonia.StyledProperty<System.Double> Avalonia.Controls.NumericUpDown.MinimumProperty' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public System.Double Avalonia.Controls.NumericUpDown.Increment.get()' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public void Avalonia.Controls.NumericUpDown.Increment.set(System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public System.Double Avalonia.Controls.NumericUpDown.Maximum.get()' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public void Avalonia.Controls.NumericUpDown.Maximum.set(System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public System.Double Avalonia.Controls.NumericUpDown.Minimum.get()' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public void Avalonia.Controls.NumericUpDown.Minimum.set(System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'protected System.Double Avalonia.Controls.NumericUpDown.OnCoerceIncrement(System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'protected System.Double Avalonia.Controls.NumericUpDown.OnCoerceMaximum(System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'protected System.Double Avalonia.Controls.NumericUpDown.OnCoerceMinimum(System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'protected System.Double Avalonia.Controls.NumericUpDown.OnCoerceValue(System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'protected void Avalonia.Controls.NumericUpDown.OnIncrementChanged(System.Double, System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'protected void Avalonia.Controls.NumericUpDown.OnMaximumChanged(System.Double, System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'protected void Avalonia.Controls.NumericUpDown.OnMinimumChanged(System.Double, System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'protected void Avalonia.Controls.NumericUpDown.OnValueChanged(System.Double, System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'protected void Avalonia.Controls.NumericUpDown.RaiseValueChangedEvent(System.Double, System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public System.Double Avalonia.Controls.NumericUpDown.Value.get()' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public void Avalonia.Controls.NumericUpDown.Value.set(System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public void Avalonia.Controls.NumericUpDownValueChangedEventArgs..ctor(Avalonia.Interactivity.RoutedEvent, System.Double, System.Double)' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public System.Double Avalonia.Controls.NumericUpDownValueChangedEventArgs.NewValue.get()' does not exist in the implementation but it does exist in the contract.
-MembersMustExist : Member 'public System.Double Avalonia.Controls.NumericUpDownValueChangedEventArgs.OldValue.get()' does not exist in the implementation but it does exist in the contract.
+MembersMustExist : Member 'public Avalonia.StyledProperty<System.Boolean> Avalonia.StyledProperty<System.Boolean> Avalonia.Controls.ScrollViewer.AllowAutoHideProperty' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public Avalonia.AvaloniaProperty<Avalonia.Media.Stretch> Avalonia.AvaloniaProperty<Avalonia.Media.Stretch> Avalonia.Controls.Viewbox.StretchProperty' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public void Avalonia.Controls.Embedding.Offscreen.OffscreenTopLevelImplBase.SetCursor(Avalonia.Platform.IPlatformHandle)' does not exist in the implementation but it does exist in the contract.
 MembersMustExist : Member 'public Avalonia.AvaloniaProperty Avalonia.AvaloniaProperty Avalonia.Controls.Notifications.NotificationCard.CloseOnClickProperty' does not exist in the implementation but it does exist in the contract.
@@ -35,4 +12,6 @@ EnumValuesMustMatch : Enum value 'Avalonia.Platform.ExtendClientAreaChromeHints
 InterfacesShouldHaveSameMembers : Interface member 'public void Avalonia.Platform.ITopLevelImpl.SetCursor(Avalonia.Platform.ICursorImpl)' is present in the implementation but not in the contract.
 InterfacesShouldHaveSameMembers : Interface member 'public void Avalonia.Platform.ITopLevelImpl.SetCursor(Avalonia.Platform.IPlatformHandle)' is present in the contract but not in the implementation.
 MembersMustExist : Member 'public void Avalonia.Platform.ITopLevelImpl.SetCursor(Avalonia.Platform.IPlatformHandle)' does not exist in the implementation but it does exist in the contract.
-Total Issues: 36
+InterfacesShouldHaveSameMembers : Interface member 'public System.Nullable<Avalonia.Size> Avalonia.Platform.ITopLevelImpl.FrameSize' is present in the implementation but not in the contract.
+InterfacesShouldHaveSameMembers : Interface member 'public System.Nullable<Avalonia.Size> Avalonia.Platform.ITopLevelImpl.FrameSize.get()' is present in the implementation but not in the contract.
+Total Issues: 15

src/Avalonia.Controls/Application.cs

@@ -13,6 +13,7 @@ using Avalonia.Platform;
 using Avalonia.Rendering;
 using Avalonia.Styling;
 using Avalonia.Threading;
+#nullable enable
 
 namespace Avalonia
 {
@@ -35,27 +36,27 @@ namespace Avalonia
         /// <summary>
         /// The application-global data templates.
         /// </summary>
-        private DataTemplates _dataTemplates;
+        private DataTemplates? _dataTemplates;
 
         private readonly Lazy<IClipboard> _clipboard =
             new Lazy<IClipboard>(() => (IClipboard)AvaloniaLocator.Current.GetService(typeof(IClipboard)));
         private readonly Styler _styler = new Styler();
-        private Styles _styles;
-        private IResourceDictionary _resources;
+        private Styles? _styles;
+        private IResourceDictionary? _resources;
         private bool _notifyingResourcesChanged;
-        private Action<IReadOnlyList<IStyle>> _stylesAdded;
-        private Action<IReadOnlyList<IStyle>> _stylesRemoved;
+        private Action<IReadOnlyList<IStyle>>? _stylesAdded;
+        private Action<IReadOnlyList<IStyle>>? _stylesRemoved;
 
         /// <summary>
         /// Defines the <see cref="DataContext"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> DataContextProperty =
+        public static readonly StyledProperty<object?> DataContextProperty =
             StyledElement.DataContextProperty.AddOwner<Application>();
 
         /// <inheritdoc/>
-        public event EventHandler<ResourcesChangedEventArgs> ResourcesChanged;
+        public event EventHandler<ResourcesChangedEventArgs>? ResourcesChanged;
 
-        public event EventHandler<UrlOpenedEventArgs> UrlsOpened; 
+        public event EventHandler<UrlOpenedEventArgs>? UrlsOpened; 
 
         /// <summary>
         /// Creates an instance of the <see cref="Application"/> class.
@@ -72,7 +73,7 @@ namespace Avalonia
         /// The data context property specifies the default object that will
         /// be used for data binding.
         /// </remarks>
-        public object DataContext
+        public object? DataContext
         {
             get { return GetValue(DataContextProperty); }
             set { SetValue(DataContextProperty, value); }
@@ -162,7 +163,7 @@ namespace Avalonia
         /// <summary>
         /// Gets the styling parent of the application, which is null.
         /// </summary>
-        IStyleHost IStyleHost.StylingParent => null;
+        IStyleHost? IStyleHost.StylingParent => null;
 
         /// <inheritdoc/>
         bool IStyleHost.IsStylesInitialized => _styles != null;
@@ -194,7 +195,7 @@ namespace Avalonia
         public virtual void Initialize() { }
 
         /// <inheritdoc/>
-        bool IResourceNode.TryGetResource(object key, out object value)
+        bool IResourceNode.TryGetResource(object key, out object? value)
         {
             value = null;
             return (_resources?.TryGetResource(key, out value) ?? false) ||
@@ -279,17 +280,17 @@ namespace Avalonia
             NotifyResourcesChanged(e);
         }
 
-        private string _name;
+        private string? _name;
         /// <summary>
         /// Defines Name property
         /// </summary>
-        public static readonly DirectProperty<Application, string> NameProperty =
-            AvaloniaProperty.RegisterDirect<Application, string>("Name", o => o.Name, (o, v) => o.Name = v);
+        public static readonly DirectProperty<Application, string?> NameProperty =
+            AvaloniaProperty.RegisterDirect<Application, string?>("Name", o => o.Name, (o, v) => o.Name = v);
 
         /// <summary>
         /// Application name to be used for various platform-specific purposes
         /// </summary>
-        public string Name
+        public string? Name
         {
             get => _name;
             set => SetAndRaise(NameProperty, ref _name, value);

src/Avalonia.Controls/ComboBox.cs

@@ -77,14 +77,6 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
             ContentControl.VerticalContentAlignmentProperty.AddOwner<ComboBox>();
 
-        /// <summary>
-        /// Defines the <see cref="IsTextSearchEnabled"/> property.
-        /// </summary>
-        public static readonly StyledProperty<bool> IsTextSearchEnabledProperty =
-            AvaloniaProperty.Register<ComboBox, bool>(nameof(IsTextSearchEnabled), true);
-
-        private string _textSearchTerm = string.Empty;
-        private DispatcherTimer _textSearchTimer;
         private bool _isDropDownOpen;
         private Popup _popup;
         private object _selectionBoxItem;
@@ -173,15 +165,6 @@ namespace Avalonia.Controls
             set { SetValue(VerticalContentAlignmentProperty, value); }
         }
 
-        /// <summary>
-        /// Gets or sets a value that specifies whether a user can jump to a value by typing.
-        /// </summary>
-        public bool IsTextSearchEnabled
-        {
-            get { return GetValue(IsTextSearchEnabledProperty); }
-            set { SetValue(IsTextSearchEnabledProperty, value); }
-        }
-
         /// <inheritdoc/>
         protected override IItemContainerGenerator CreateItemContainerGenerator()
         {
@@ -205,7 +188,7 @@ namespace Avalonia.Controls
             if (e.Handled)
                 return;
 
-            if (e.Key == Key.F4 ||
+            if ((e.Key == Key.F4 && e.KeyModifiers.HasAllFlags(KeyModifiers.Alt) == false) ||
                 ((e.Key == Key.Down || e.Key == Key.Up) && e.KeyModifiers.HasAllFlags(KeyModifiers.Alt)))
             {
                 IsDropDownOpen = !IsDropDownOpen;
@@ -247,32 +230,6 @@ namespace Avalonia.Controls
             }
         }
 
-        /// <inheritdoc />
-        protected override void OnTextInput(TextInputEventArgs e)
-        {
-            if (!IsTextSearchEnabled || e.Handled)
-                return;
-
-            StopTextSearchTimer();
-
-            _textSearchTerm += e.Text;
-
-            bool match(ItemContainerInfo info) => 
-                info.ContainerControl is IContentControl control &&
-                control.Content?.ToString()?.StartsWith(_textSearchTerm, StringComparison.OrdinalIgnoreCase) == true;
-
-            var info = ItemContainerGenerator.Containers.FirstOrDefault(match);
-
-            if (info != null)
-            {
-                SelectedIndex = info.Index;
-            }
-
-            StartTextSearchTimer();
-
-            e.Handled = true;
-        }
-
         /// <inheritdoc/>
         protected override void OnPointerWheelChanged(PointerWheelEventArgs e)
         {
@@ -470,31 +427,5 @@ namespace Avalonia.Controls
 
             SelectedIndex = prev;
         }
-
-        private void StartTextSearchTimer()
-        {
-            _textSearchTimer = new DispatcherTimer { Interval = TimeSpan.FromSeconds(1) };
-            _textSearchTimer.Tick += TextSearchTimer_Tick;
-            _textSearchTimer.Start();
-        }
-
-        private void StopTextSearchTimer()
-        {
-            if (_textSearchTimer == null)
-            {
-                return;
-            }
-
-            _textSearchTimer.Stop();
-            _textSearchTimer.Tick -= TextSearchTimer_Tick;
-
-            _textSearchTimer = null;
-        }
-
-        private void TextSearchTimer_Tick(object sender, EventArgs e)
-        {
-            _textSearchTerm = string.Empty;
-            StopTextSearchTimer();
-        }
     }
 }

src/Avalonia.Controls/DateTimePickers/DatePicker.cs

@@ -2,6 +2,7 @@
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
+using Avalonia.Data;
 using Avalonia.Interactivity;
 using System;
 using System.Collections.Generic;
@@ -88,7 +89,8 @@ namespace Avalonia.Controls
         /// </summary>
         public static readonly DirectProperty<DatePicker, DateTimeOffset?> SelectedDateProperty =
             AvaloniaProperty.RegisterDirect<DatePicker, DateTimeOffset?>(nameof(SelectedDate), 
-                x => x.SelectedDate, (x, v) => x.SelectedDate = v);
+                x => x.SelectedDate, (x, v) => x.SelectedDate = v,
+                defaultBindingMode: BindingMode.TwoWay);
 
         // Template Items
         private Button _flyoutButton;

src/Avalonia.Controls/DateTimePickers/TimePicker.cs

@@ -2,6 +2,7 @@
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
+using Avalonia.Data;
 using System;
 using System.Globalization;
 
@@ -44,7 +45,8 @@ namespace Avalonia.Controls
         /// </summary>
         public static readonly DirectProperty<TimePicker, TimeSpan?> SelectedTimeProperty =
             AvaloniaProperty.RegisterDirect<TimePicker, TimeSpan?>(nameof(SelectedTime),
-                x => x.SelectedTime, (x, v) => x.SelectedTime = v);
+                x => x.SelectedTime, (x, v) => x.SelectedTime = v,
+                defaultBindingMode: BindingMode.TwoWay);
 
         // Template Items
         private TimePickerPresenter _presenter;

src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevelImpl.cs

@@ -35,6 +35,8 @@ namespace Avalonia.Controls.Embedding.Offscreen
             }
         }
 
+        public Size? FrameSize => null;
+
         public double RenderScaling
         {
             get { return _scaling; }

src/Avalonia.Controls/Expander.cs

@@ -1,7 +1,11 @@
+using System.Threading;
+
 using Avalonia.Animation;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
 
+#nullable enable
+
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -36,8 +40,8 @@ namespace Avalonia.Controls
     [PseudoClasses(":expanded", ":up", ":down", ":left", ":right")]
     public class Expander : HeaderedContentControl
     {
-        public static readonly StyledProperty<IPageTransition> ContentTransitionProperty =
-            AvaloniaProperty.Register<Expander, IPageTransition>(nameof(ContentTransition));
+        public static readonly StyledProperty<IPageTransition?> ContentTransitionProperty =
+            AvaloniaProperty.Register<Expander, IPageTransition?>(nameof(ContentTransition));
 
         public static readonly StyledProperty<ExpandDirection> ExpandDirectionProperty =
             AvaloniaProperty.Register<Expander, ExpandDirection>(nameof(ExpandDirection), ExpandDirection.Down);
@@ -50,6 +54,7 @@ namespace Avalonia.Controls
                 defaultBindingMode: Data.BindingMode.TwoWay);
 
         private bool _isExpanded;
+        private CancellationTokenSource? _lastTransitionCts;
 
         static Expander()
         {
@@ -61,7 +66,7 @@ namespace Avalonia.Controls
             UpdatePseudoClasses(ExpandDirection);
         }
 
-        public IPageTransition ContentTransition
+        public IPageTransition? ContentTransition
         {
             get => GetValue(ContentTransitionProperty);
             set => SetValue(ContentTransitionProperty, value);
@@ -83,19 +88,23 @@ namespace Avalonia.Controls
             }
         }
 
-        protected virtual void OnIsExpandedChanged(AvaloniaPropertyChangedEventArgs e)
+        protected virtual async void OnIsExpandedChanged(AvaloniaPropertyChangedEventArgs e)
         {
             if (Content != null && ContentTransition != null && Presenter is Visual visualContent)
             {
                 bool forward = ExpandDirection == ExpandDirection.Left ||
                                 ExpandDirection == ExpandDirection.Up;
+
+                _lastTransitionCts?.Cancel();
+                _lastTransitionCts = new CancellationTokenSource();
+
                 if (IsExpanded)
                 {
-                    ContentTransition.Start(null, visualContent, forward);
+                    await ContentTransition.Start(null, visualContent, forward, _lastTransitionCts.Token);
                 }
                 else
                 {
-                    ContentTransition.Start(visualContent, null, !forward);
+                    await ContentTransition.Start(visualContent, null, forward, _lastTransitionCts.Token);
                 }
             }
         }

src/Avalonia.Controls/ExperimentalAcrylicBorder.cs

@@ -3,6 +3,7 @@ using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.Platform;
 using System;
+using Avalonia.Media.Immutable;
 
 namespace Avalonia.Controls
 {
@@ -90,7 +91,7 @@ namespace Avalonia.Controls
             }
             else
             {
-                _borderRenderHelper.Render(context, Bounds.Size, new Thickness(), CornerRadius, new SolidColorBrush(Material.FallbackColor), null, default);
+                _borderRenderHelper.Render(context, Bounds.Size, new Thickness(), CornerRadius, new ImmutableSolidColorBrush(Material.FallbackColor), null, default);
             }
         }
 

src/Avalonia.Controls/Flyouts/FlyoutBase.cs

@@ -4,6 +4,7 @@ using Avalonia.Input;
 using Avalonia.Input.Raw;
 using Avalonia.Layout;
 using Avalonia.Logging;
+using Avalonia.Rendering;
 
 #nullable enable
 
@@ -51,8 +52,9 @@ namespace Avalonia.Controls.Primitives
         private bool _isOpen;
         private Control? _target;
         private FlyoutShowMode _showMode = FlyoutShowMode.Standard;
-        private Rect? enlargedPopupRect;
-        private IDisposable? transientDisposable;
+        private Rect? _enlargedPopupRect;
+        private PixelRect? _enlargePopupRectScreenPixelRect;
+        private IDisposable? _transientDisposable;
 
         protected Popup? Popup { get; private set; }
 
@@ -163,8 +165,10 @@ namespace Avalonia.Controls.Primitives
             Popup.IsOpen = false;
 
             // Ensure this isn't active
-            transientDisposable?.Dispose();
-            transientDisposable = null;
+            _transientDisposable?.Dispose();
+            _transientDisposable = null;
+            _enlargedPopupRect = null;
+            _enlargePopupRectScreenPixelRect = null;
 
             OnClosed();
         }
@@ -230,7 +234,7 @@ namespace Avalonia.Controls.Primitives
             }
             else if (ShowMode == FlyoutShowMode.TransientWithDismissOnPointerMoveAway)
             {
-                transientDisposable = InputManager.Instance?.Process.Subscribe(HandleTransientDismiss);
+                _transientDisposable = InputManager.Instance?.Process.Subscribe(HandleTransientDismiss);
             }
         }
 
@@ -246,20 +250,20 @@ namespace Avalonia.Controls.Primitives
                 // For windowed popups, enlargedPopupRect is in screen coordinates,
                 // for overlay popups, its in OverlayLayer coordinates
 
-                if (enlargedPopupRect == null)
+                if (_enlargedPopupRect == null && _enlargePopupRectScreenPixelRect == null)
                 {
                     // Only do this once when the Flyout opens & cache the result
                     if (Popup?.Host is PopupRoot root)
                     { 
                         // Get the popup root bounds and convert to screen coordinates
+                        
                         var tmp = root.Bounds.Inflate(100);
-                        var scPt = root.PointToScreen(tmp.TopLeft);
-                        enlargedPopupRect = new Rect(scPt.X, scPt.Y, tmp.Width, tmp.Height);
+                        _enlargePopupRectScreenPixelRect = new PixelRect(root.PointToScreen(tmp.TopLeft), root.PointToScreen(tmp.BottomRight));
                     }
                     else if (Popup?.Host is OverlayPopupHost host)
                     {
                         // Overlay popups are in OverlayLayer coordinates, just use that
-                        enlargedPopupRect = host.Bounds.Inflate(100);
+                        _enlargedPopupRect = host.Bounds.Inflate(100);
                     }
 
                     return;
@@ -273,24 +277,18 @@ namespace Avalonia.Controls.Primitives
                     // window will not close this (as pointer events stop), which 
                     // does match UWP
                     var pt = pArgs.Root.PointToScreen(pArgs.Position);
-                    if (!enlargedPopupRect?.Contains(new Point(pt.X, pt.Y)) ?? false)
+                    if (!_enlargePopupRectScreenPixelRect?.Contains(pt) ?? false)
                     {
                         HideCore(false);
-                        enlargedPopupRect = null;
-                        transientDisposable?.Dispose();
-                        transientDisposable = null;
                     }
                 }
                 else if (Popup?.Host is OverlayPopupHost)
                 {
                     // Same as above here, but just different coordinate space
                     // so we don't need to translate
-                    if (!enlargedPopupRect?.Contains(pArgs.Position) ?? false)
+                    if (!_enlargedPopupRect?.Contains(pArgs.Position) ?? false)
                     {
                         HideCore(false);
-                        enlargedPopupRect = null;
-                        transientDisposable?.Dispose();
-                        transientDisposable = null;
                     }
                 }
             }

src/Avalonia.Controls/Grid.cs

@@ -978,6 +978,7 @@ namespace Avalonia.Controls
         /// width is not registered in columns.</param>
         /// <param name="forceInfinityV">Passed through to MeasureCell.
         /// When "true" cells' desired height is not registered in rows.</param>
+        /// <param name="hasDesiredSizeUChanged">return true when desired size has changed</param>
         private void MeasureCellsGroup(
             int cellsHead,
             Size referenceSize,

src/Avalonia.Controls/GridLength.cs

@@ -77,6 +77,12 @@ namespace Avalonia.Controls
         /// </summary>
         public static GridLength Auto => new GridLength(0, GridUnitType.Auto);
 
+        /// <summary>
+        /// Gets an instance of <see cref="GridLength"/> that indicates that a row or column should
+        /// fill its content.
+        /// </summary>
+        public static GridLength Star => new GridLength(1, GridUnitType.Star);
+
         /// <summary>
         /// Gets the unit of the <see cref="GridLength"/>.
         /// </summary>

src/Avalonia.Controls/ItemsControl.cs

@@ -2,6 +2,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Linq;
 using Avalonia.Collections;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Metadata;

src/Avalonia.Controls/Menu.cs

@@ -17,7 +17,6 @@ namespace Avalonia.Controls
         private static readonly ITemplate<IPanel> DefaultPanel =
             new FuncTemplate<IPanel>(() => new StackPanel { Orientation = Orientation.Horizontal });
 
-        private LightDismissOverlayLayer? _overlay;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Menu"/> class.

src/Avalonia.Controls/MenuItem.cs

@@ -36,7 +36,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="HotKey"/> property.
         /// </summary>
-        public static readonly StyledProperty<KeyGesture> HotKeyProperty =
+        public static readonly StyledProperty<KeyGesture?> HotKeyProperty =
             HotKeyManager.HotKeyProperty.AddOwner<MenuItem>();
 
         /// <summary>
@@ -108,7 +108,7 @@ namespace Avalonia.Controls
         private ICommand? _command;
         private bool _commandCanExecute = true;
         private Popup? _popup;
-        private KeyGesture _hotkey;
+        private KeyGesture? _hotkey;
         private bool _isEmbeddedInMenu;
 
         /// <summary>
@@ -214,7 +214,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets an <see cref="KeyGesture"/> associated with this control
         /// </summary>
-        public KeyGesture HotKey
+        public KeyGesture? HotKey
         {
             get { return GetValue(HotKeyProperty); }
             set { SetValue(HotKeyProperty, value); }

src/Avalonia.Controls/Platform/ITopLevelImpl.cs

@@ -22,6 +22,11 @@ namespace Avalonia.Platform
         /// </summary>
         Size ClientSize { get; }
 
+        /// <summary>
+        /// Gets the total size of the toplevel, excluding shadows.
+        /// </summary>
+        Size? FrameSize { get; }
+
         /// <summary>
         /// Gets the scaling factor for the toplevel. This is used for rendering.
         /// </summary>

src/Avalonia.Controls/Presenters/CarouselPresenter.cs

@@ -186,7 +186,7 @@ namespace Avalonia.Controls.Presenters
 
                 if (PageTransition != null && (from != null || to != null))
                 {
-                    await PageTransition.Start((Visual)from, (Visual)to, fromIndex < toIndex);
+                    await PageTransition.Start((Visual)from, (Visual)to, fromIndex < toIndex, default);
                 }
                 else if (to != null)
                 {

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -511,8 +511,8 @@ namespace Avalonia.Controls.Presenters
             else if (scrollable.IsLogicalScrollEnabled)
             {
                 Viewport = scrollable.Viewport;
-                Extent = scrollable.Extent;
                 Offset = scrollable.Offset;
+                Extent = scrollable.Extent;
             }
         }
 

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -6,6 +6,7 @@ using Avalonia.Metadata;
 using Avalonia.Threading;
 using Avalonia.VisualTree;
 using Avalonia.Layout;
+using Avalonia.Media.Immutable;
 
 namespace Avalonia.Controls.Presenters
 {
@@ -360,32 +361,31 @@ namespace Avalonia.Controls.Presenters
 
             RenderInternal(context);
 
-            if (selectionStart == selectionEnd)
+            if (selectionStart == selectionEnd && _caretBlink)
             {
-                var caretBrush = CaretBrush;
+                var caretBrush = CaretBrush?.ToImmutable();
 
                 if (caretBrush is null)
                 {
-                    var backgroundColor = (Background as SolidColorBrush)?.Color;
+                    var backgroundColor = (Background as ISolidColorBrush)?.Color;
                     if (backgroundColor.HasValue)
                     {
                         byte red = (byte)~(backgroundColor.Value.R);
                         byte green = (byte)~(backgroundColor.Value.G);
                         byte blue = (byte)~(backgroundColor.Value.B);
 
-                        caretBrush = new SolidColorBrush(Color.FromRgb(red, green, blue));
+                        caretBrush = new ImmutableSolidColorBrush(Color.FromRgb(red, green, blue));
                     }
                     else
+                    {
                         caretBrush = Brushes.Black;
+                    }
                 }
 
-                if (_caretBlink)
-                {
-                    var (p1, p2) = GetCaretPoints();
-                    context.DrawLine(
-                        new Pen(caretBrush, 1),
-                        p1, p2);
-                }
+                var (p1, p2) = GetCaretPoints();
+                context.DrawLine(
+                    new ImmutablePen(caretBrush, 1),
+                    p1, p2);
             }
         }
 

src/Avalonia.Controls/Primitives/AdornerLayer.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Specialized;
 using System.Linq;
+using System.Resources;
 using Avalonia.Media;
 using Avalonia.Rendering;
 using Avalonia.Utilities;
@@ -45,10 +46,27 @@ namespace Avalonia.Controls.Primitives
                 ?.AdornerLayer;
         }
 
-        protected override Size ArrangeOverride(Size finalSize)
+        protected override Size MeasureOverride(Size availableSize)
         {
-            var parent = Parent;
+            foreach (var child in Children)
+            {
+                var info = child.GetValue(s_adornedElementInfoProperty);
+
+                if (info != null && info.Bounds.HasValue)
+                {
+                    child.Measure(info.Bounds.Value.Bounds.Size);
+                }
+                else
+                {
+                    child.Measure(availableSize);
+                }
+            }
 
+            return default;
+        }
+
+        protected override Size ArrangeOverride(Size finalSize)
+        {
             foreach (var child in Children)
             {
                 var info = child.GetValue(s_adornedElementInfoProperty);

src/Avalonia.Controls/Primitives/LightDismissOverlayLayer.cs

@@ -52,8 +52,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (InputPassThroughElement is object)
             {
-                var p = point.Transform(this.TransformToVisual(VisualRoot)!.Value);
-                var hit = VisualRoot.GetVisualAt(p, x => x != this);
+                var hit = VisualRoot.GetVisualAt(point, x => x != this);
 
                 if (hit is object)
                 {

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -10,6 +10,7 @@ using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
+using Avalonia.Threading;
 using Avalonia.VisualTree;
 
 #nullable enable
@@ -91,6 +92,12 @@ namespace Avalonia.Controls.Primitives
             AvaloniaProperty.Register<SelectingItemsControl, SelectionMode>(
                 nameof(SelectionMode));
 
+        /// <summary>
+        /// Defines the <see cref="IsTextSearchEnabled"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> IsTextSearchEnabledProperty =
+            AvaloniaProperty.Register<ItemsControl, bool>(nameof(IsTextSearchEnabled), true);
+
         /// <summary>
         /// Event that should be raised by items that implement <see cref="ISelectable"/> to
         /// notify the parent <see cref="SelectingItemsControl"/> that their selection state
@@ -110,6 +117,8 @@ namespace Avalonia.Controls.Primitives
                 RoutingStrategies.Bubble);
 
         private static readonly IList Empty = Array.Empty<object>();
+        private string _textSearchTerm = string.Empty;
+        private DispatcherTimer? _textSearchTimer;
         private ISelectionModel? _selection;
         private int _oldSelectedIndex;
         private object? _oldSelectedItem;
@@ -305,6 +314,15 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <summary>
+        /// Gets or sets a value that specifies whether a user can jump to a value by typing.
+        /// </summary>
+        public bool IsTextSearchEnabled
+        {
+            get { return GetValue(IsTextSearchEnabledProperty); }
+            set { SetValue(IsTextSearchEnabledProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets the selection mode.
         /// </summary>
@@ -490,6 +508,36 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        protected override void OnTextInput(TextInputEventArgs e)
+        {
+            if (!e.Handled)
+            {
+                if (!IsTextSearchEnabled)
+                    return;
+
+                StopTextSearchTimer();
+
+                _textSearchTerm += e.Text;
+
+                bool match(ItemContainerInfo info) =>
+                    info.ContainerControl is IContentControl control &&
+                    control.Content?.ToString()?.StartsWith(_textSearchTerm, StringComparison.OrdinalIgnoreCase) == true;
+
+                var info = ItemContainerGenerator.Containers.FirstOrDefault(match);
+
+                if (info != null)
+                {
+                    SelectedIndex = info.Index;
+                }
+
+                StartTextSearchTimer();
+
+                e.Handled = true;
+            }
+
+            base.OnTextInput(e);
+        }
+
         protected override void OnKeyDown(KeyEventArgs e)
         {
             base.OnKeyDown(e);
@@ -962,6 +1010,32 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        private void StartTextSearchTimer()
+        {
+            _textSearchTimer = new DispatcherTimer { Interval = TimeSpan.FromSeconds(1) };
+            _textSearchTimer.Tick += TextSearchTimer_Tick;
+            _textSearchTimer.Start();
+        }
+
+        private void StopTextSearchTimer()
+        {
+            if (_textSearchTimer == null)
+            {
+                return;
+            }
+
+            _textSearchTimer.Tick -= TextSearchTimer_Tick;
+            _textSearchTimer.Stop();
+
+            _textSearchTimer = null;
+        }
+
+        private void TextSearchTimer_Tick(object sender, EventArgs e)
+        {
+            _textSearchTerm = string.Empty;
+            StopTextSearchTimer();
+        }
+
         // When in a BeginInit..EndInit block, or when the DataContext is updating, we need to
         // defer changes to the selection model because we have no idea in which order properties
         // will be set. Consider:

src/Avalonia.Controls/Remote/RemoteWidget.cs

@@ -70,7 +70,7 @@ namespace Avalonia.Controls.Remote
 
         public override void Render(DrawingContext context)
         {
-            if (_lastFrame != null)
+            if (_lastFrame != null && _lastFrame.Width != 0 && _lastFrame.Height != 0)
             {
                 var fmt = (PixelFormat) _lastFrame.Format;
                 if (_bitmap == null || _bitmap.PixelSize.Width != _lastFrame.Width ||

src/Avalonia.Controls/Repeater/ViewportManager.cs

@@ -27,7 +27,6 @@ namespace Avalonia.Controls
         private IScrollAnchorProvider _scroller;
         private IControl _makeAnchorElement;
         private bool _isAnchorOutsideRealizedRange;
-        private Task _cacheBuildAction;
         private Rect _visibleWindow;
         private Rect _layoutExtent;
         // This is the expected shift by the layout.

src/Avalonia.Controls/Repeater/VirtualizationInfo.cs

@@ -27,7 +27,6 @@ namespace Avalonia.Controls
     internal class VirtualizationInfo
     {
         private int _pinCounter;
-        private object _data;
 
         public Rect ArrangeBounds { get; set; }
         public bool AutoRecycleCandidate { get; set; }

src/Avalonia.Controls/RowDefinitions.cs

@@ -1,5 +1,4 @@
 using System.Linq;
-using Avalonia.Collections;
 
 namespace Avalonia.Controls
 {
@@ -25,6 +24,11 @@ namespace Avalonia.Controls
             AddRange(GridLength.ParseLengths(s).Select(x => new RowDefinition(x)));
         }
 
+        public override string ToString()
+        {
+            return string.Join(",", this.Select(x => x.Height));
+        }
+
         /// <summary>
         /// Parses a string representation of row definitions collection.
         /// </summary>
@@ -32,4 +36,4 @@ namespace Avalonia.Controls
         /// <returns>The <see cref="RowDefinitions"/>.</returns>
         public static RowDefinitions Parse(string s) => new RowDefinitions(s);
     }
-}
+}

src/Avalonia.Controls/ScrollViewer.cs

@@ -176,8 +176,10 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="AllowAutoHide"/> property.
         /// </summary>
-        public static readonly StyledProperty<bool> AllowAutoHideProperty =
-            ScrollBar.AllowAutoHideProperty.AddOwner<ScrollViewer>();
+        public static readonly AttachedProperty<bool> AllowAutoHideProperty =
+            AvaloniaProperty.RegisterAttached<ScrollViewer, Control, bool>(
+                nameof(AllowAutoHide),
+                true);
 
         /// <summary>
         /// Defines the <see cref="ScrollChanged"/> event.
@@ -207,8 +209,8 @@ namespace Avalonia.Controls
         /// </summary>
         static ScrollViewer()
         {
-            HorizontalScrollBarVisibilityProperty.Changed.AddClassHandler<ScrollViewer>((x, e) => x.ScrollBarVisibilityChanged(e));
-            VerticalScrollBarVisibilityProperty.Changed.AddClassHandler<ScrollViewer>((x, e) => x.ScrollBarVisibilityChanged(e));
+            HorizontalScrollBarVisibilityProperty.Changed.AddClassHandler<ScrollViewer, ScrollBarVisibility>((x, e) => x.ScrollBarVisibilityChanged(e));
+            VerticalScrollBarVisibilityProperty.Changed.AddClassHandler<ScrollViewer, ScrollBarVisibility>((x, e) => x.ScrollBarVisibilityChanged(e));
         }
 
         /// <summary>
@@ -526,6 +528,26 @@ namespace Avalonia.Controls
             return control.GetValue(VerticalScrollBarVisibilityProperty);
         }
 
+        /// <summary>
+        /// Gets the value of the AllowAutoHideProperty attached property.
+        /// </summary>
+        /// <param name="control">The control to set the value on.</param>
+        /// <param name="value">The value of the property.</param>
+        public static void SetAllowAutoHide(Control control, bool value)
+        {
+            control.SetValue(AllowAutoHideProperty, value);
+        }
+
+        /// <summary>
+        /// Gets the value of the AllowAutoHideProperty attached property.
+        /// </summary>
+        /// <param name="control">The control to read the value from.</param>
+        /// <returns>The value of the property.</returns>
+        public static bool GetAllowAutoHide(Control control)
+        {
+            return control.GetValue(AllowAutoHideProperty);
+        }
+
         /// <summary>
         /// Gets the value of the VerticalScrollBarVisibility attached property.
         /// </summary>
@@ -604,10 +626,10 @@ namespace Avalonia.Controls
             CalculatedPropertiesChanged();
         }
 
-        private void ScrollBarVisibilityChanged(AvaloniaPropertyChangedEventArgs e)
+        private void ScrollBarVisibilityChanged(AvaloniaPropertyChangedEventArgs<ScrollBarVisibility> e)
         {
-            var wasEnabled = !ScrollBarVisibility.Disabled.Equals(e.OldValue);
-            var isEnabled = !ScrollBarVisibility.Disabled.Equals(e.NewValue);
+            var wasEnabled = e.OldValue.GetValueOrDefault() != ScrollBarVisibility.Disabled;
+            var isEnabled = e.NewValue.GetValueOrDefault() != ScrollBarVisibility.Disabled;
 
             if (wasEnabled != isEnabled)
             {

src/Avalonia.Controls/Shapes/Arc.cs

@@ -0,0 +1,103 @@
+using System;
+using Avalonia.Media;
+
+namespace Avalonia.Controls.Shapes
+{
+    public class Arc : Shape
+    {
+        /// <summary>
+        /// Defines the <see cref="StartAngle"/> property.
+        /// </summary>
+        public static readonly StyledProperty<double> StartAngleProperty =
+            AvaloniaProperty.Register<Arc, double>(nameof(StartAngle), 0.0);
+
+        /// <summary>
+        /// Defines the <see cref="SweepAngle"/> property.
+        /// </summary>
+        public static readonly StyledProperty<double> SweepAngleProperty =
+            AvaloniaProperty.Register<Arc, double>(nameof(SweepAngle), 0.0);
+
+        static Arc()
+        {
+            StrokeThicknessProperty.OverrideDefaultValue<Arc>(1);
+            AffectsGeometry<Arc>(BoundsProperty, StrokeThicknessProperty, StartAngleProperty, SweepAngleProperty);
+        }
+
+        /// <summary>
+        /// Gets or sets the angle at which the arc starts, in degrees.
+        /// </summary>
+        public double StartAngle
+        {
+            get => GetValue(StartAngleProperty);
+            set => SetValue(StartAngleProperty, value);
+        }
+
+        /// <summary>
+        /// Gets or sets the angle, in degrees, added to the <see cref="StartAngle"/> defining where the arc ends.
+        /// A positive value is clockwise, negative is counter-clockwise.
+        /// </summary>
+        public double SweepAngle
+        {
+            get => GetValue(SweepAngleProperty);
+            set => SetValue(SweepAngleProperty, value);
+        }
+
+        protected override Geometry CreateDefiningGeometry()
+        {
+            var angle1 = DegreesToRad(StartAngle);
+            var angle2 = angle1 + DegreesToRad(SweepAngle);
+
+            var startAngle = Math.Min(angle1, angle2);
+            var sweepAngle = Math.Max(angle1, angle2);
+
+            var normStart = RadToNormRad(startAngle);
+            var normEnd = RadToNormRad(sweepAngle);
+
+            var rect = new Rect(Bounds.Size);
+
+            if ((normStart == normEnd) && (startAngle != sweepAngle)) // Complete ring.
+            {
+                return new EllipseGeometry(rect.Deflate(StrokeThickness / 2));
+            }
+            else if (SweepAngle == 0)
+            {
+                return new StreamGeometry();
+            }
+            else // Partial arc.
+            {
+                var deflatedRect = rect.Deflate(StrokeThickness / 2);
+
+                var centerX = rect.Center.X;
+                var centerY = rect.Center.Y;
+
+                var radiusX = deflatedRect.Width / 2;
+                var radiusY = deflatedRect.Height / 2;
+
+                var angleGap = RadToNormRad(sweepAngle - startAngle);
+
+                var startPoint = GetRingPoint(radiusX, radiusY, centerX, centerY, startAngle);
+                var endPoint = GetRingPoint(radiusX, radiusY, centerX, centerY, sweepAngle);
+
+                var arcGeometry = new StreamGeometry();
+
+                using (var ctx = arcGeometry.Open())
+                {
+                    ctx.BeginFigure(startPoint, false);
+                    ctx.ArcTo(endPoint, new Size(radiusX, radiusY), angleGap, angleGap >= Math.PI,
+                        SweepDirection.Clockwise);
+                    ctx.EndFigure(false);
+                }
+
+                return arcGeometry;
+            }
+        }
+
+        static double DegreesToRad(double inAngle) =>
+            inAngle * Math.PI / 180;
+
+        static double RadToNormRad(double inAngle) => ((inAngle % (Math.PI * 2)) + (Math.PI * 2)) % (Math.PI * 2);
+
+        static Point GetRingPoint(double radiusX, double radiusY, double centerX, double centerY, double angle) =>
+            new Point((radiusX * Math.Cos(angle)) + centerX, (radiusY * Math.Sin(angle)) + centerY);
+    }
+}

src/Avalonia.Controls/Shapes/Shape.cs

@@ -1,6 +1,7 @@
 using System;
 using Avalonia.Collections;
 using Avalonia.Media;
+using Avalonia.Media.Immutable;
 
 #nullable enable
 
@@ -199,8 +200,29 @@ namespace Avalonia.Controls.Shapes
 
             if (geometry != null)
             {
-                var pen = new Pen(Stroke, StrokeThickness, new DashStyle(StrokeDashArray, StrokeDashOffset),
-                     StrokeLineCap, StrokeJoin);
+                var stroke = Stroke;
+
+                ImmutablePen? pen = null;
+
+                if (stroke != null)
+                {
+                    var strokeDashArray = StrokeDashArray;
+
+                    ImmutableDashStyle? dashStyle = null;
+
+                    if (strokeDashArray != null && strokeDashArray.Count > 0)
+                    {
+                        dashStyle = new ImmutableDashStyle(strokeDashArray, StrokeDashOffset);
+                    }
+
+                    pen = new ImmutablePen(
+                        stroke.ToImmutable(),
+                        StrokeThickness,
+                        dashStyle,
+                        StrokeLineCap,
+                        StrokeJoin);
+                }
+
                 context.DrawGeometry(Fill, pen, geometry);
             }
         }

src/Avalonia.Controls/SplitView.cs

@@ -133,7 +133,7 @@ namespace Avalonia.Controls
             AvaloniaProperty.Register<SplitView, object?>(nameof(Pane));
 
         /// <summary>
-        /// Defines the <see cref="HeaderTemplate"/> property.
+        /// Defines the <see cref="PaneTemplate"/> property.
         /// </summary>
         public static readonly StyledProperty<IDataTemplate?> PaneTemplateProperty =
             AvaloniaProperty.Register<HeaderedContentControl, IDataTemplate?>(nameof(PaneTemplate));

src/Avalonia.Controls/TextBox.cs

@@ -119,9 +119,9 @@ namespace Avalonia.Controls
             AvaloniaProperty.Register<TextBox, bool>(nameof(RevealPassword));
         
         public static readonly DirectProperty<TextBox, bool> CanCutProperty =
-                    AvaloniaProperty.RegisterDirect<TextBox, bool>(
-                        nameof(CanCut),
-                        o => o.CanCut);
+            AvaloniaProperty.RegisterDirect<TextBox, bool>(
+                nameof(CanCut),
+                o => o.CanCut);
 
         public static readonly DirectProperty<TextBox, bool> CanCopyProperty =
             AvaloniaProperty.RegisterDirect<TextBox, bool>(
@@ -129,11 +129,23 @@ namespace Avalonia.Controls
                 o => o.CanCopy);
 
         public static readonly DirectProperty<TextBox, bool> CanPasteProperty =
-                    AvaloniaProperty.RegisterDirect<TextBox, bool>(
-                        nameof(CanPaste),
-                        o => o.CanPaste);
+            AvaloniaProperty.RegisterDirect<TextBox, bool>(
+                nameof(CanPaste),
+                o => o.CanPaste);
 
-        struct UndoRedoState : IEquatable<UndoRedoState>
+        public static readonly StyledProperty<bool> IsUndoEnabledProperty =
+            AvaloniaProperty.Register<TextBox, bool>(
+                nameof(IsUndoEnabled), 
+                defaultValue: true);
+
+        public static readonly DirectProperty<TextBox, int> UndoLimitProperty =
+            AvaloniaProperty.RegisterDirect<TextBox, int>(
+                nameof(UndoLimit),
+                o => o.UndoLimit,
+                (o, v) => o.UndoLimit = v,
+                unsetValue: -1);
+
+        readonly struct UndoRedoState : IEquatable<UndoRedoState>
         {
             public string Text { get; }
             public int CaretPosition { get; }
@@ -218,7 +230,7 @@ namespace Avalonia.Controls
                 value = CoerceCaretIndex(value);
                 SetAndRaise(CaretIndexProperty, ref _caretIndex, value);
                 UndoRedoState state;
-                if (_undoRedoHelper.TryGetLastState(out state) && state.Text == Text)
+                if (IsUndoEnabled && _undoRedoHelper.TryGetLastState(out state) && state.Text == Text)
                     _undoRedoHelper.UpdateLastState();
             }
         }
@@ -316,7 +328,7 @@ namespace Avalonia.Controls
                     SelectionEnd = CoerceCaretIndex(SelectionEnd, value);
                     CaretIndex = CoerceCaretIndex(caretIndex, value);
 
-                    if (SetAndRaise(TextProperty, ref _text, value) && !_isUndoingRedoing)
+                    if (SetAndRaise(TextProperty, ref _text, value) && IsUndoEnabled && !_isUndoingRedoing)
                     {
                         _undoRedoHelper.Clear();
                     }
@@ -329,7 +341,7 @@ namespace Avalonia.Controls
             get { return GetSelection(); }
             set
             {
-                _undoRedoHelper.Snapshot();
+                SnapshotUndoRedo();
                 if (string.IsNullOrEmpty(value))
                 {
                     DeleteSelection();
@@ -338,7 +350,7 @@ namespace Avalonia.Controls
                 {
                     HandleTextInput(value);
                 }
-                _undoRedoHelper.Snapshot();
+                SnapshotUndoRedo();
             }
         }
 
@@ -446,6 +458,36 @@ namespace Avalonia.Controls
             private set { SetAndRaise(CanPasteProperty, ref _canPaste, value); }
         }
 
+        /// <summary>
+        /// Property for determining whether undo/redo is enabled
+        /// </summary>
+        public bool IsUndoEnabled
+        {
+            get { return GetValue(IsUndoEnabledProperty); }
+            set { SetValue(IsUndoEnabledProperty, value); }
+        }
+
+        public int UndoLimit
+        {
+            get { return _undoRedoHelper.Limit; }
+            set
+            {
+                if (_undoRedoHelper.Limit != value)
+                {
+                    // can't use SetAndRaise due to using _undoRedoHelper.Limit
+                    // (can't send a ref of a property to SetAndRaise),
+                    // so use RaisePropertyChanged instead.
+                    var oldValue = _undoRedoHelper.Limit;
+                    _undoRedoHelper.Limit = value;
+                    RaisePropertyChanged(UndoLimitProperty, oldValue, value);
+                } 
+                // from docs at
+                // https://docs.microsoft.com/en-us/dotnet/api/system.windows.controls.primitives.textboxbase.isundoenabled:
+                // "Setting UndoLimit clears the undo queue."
+                _undoRedoHelper.Clear();
+            }
+        }
+
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             _presenter = e.NameScope.Get<TextPresenter>("PART_TextPresenter");
@@ -465,6 +507,15 @@ namespace Avalonia.Controls
                 UpdatePseudoclasses();
                 UpdateCommandStates();
             }
+            else if (change.Property == IsUndoEnabledProperty && change.NewValue.GetValueOrDefault<bool>() == false)
+            {
+                // from docs at
+                // https://docs.microsoft.com/en-us/dotnet/api/system.windows.controls.primitives.textboxbase.isundoenabled:
+                // "Setting this property to false clears the undo stack.
+                // Therefore, if you disable undo and then re-enable it, undo commands still do not work
+                // because the undo stack was emptied when you disabled undo."
+                _undoRedoHelper.Clear();
+            }
         }
 
         private void UpdateCommandStates()
@@ -551,7 +602,10 @@ namespace Avalonia.Controls
                 SetTextInternal(text.Substring(0, caretIndex) + input + text.Substring(caretIndex));
                 CaretIndex += input.Length;
                 ClearSelection();
-                _undoRedoHelper.DiscardRedo();
+                if (IsUndoEnabled)
+                {
+                    _undoRedoHelper.DiscardRedo();
+                }
             }
         }
 
@@ -570,10 +624,10 @@ namespace Avalonia.Controls
             var text = GetSelection();
             if (text is null) return;
 
-            _undoRedoHelper.Snapshot();
+            SnapshotUndoRedo();
             Copy();
             DeleteSelection();
-            _undoRedoHelper.Snapshot();
+            SnapshotUndoRedo();
         }
 
         public async void Copy()
@@ -591,9 +645,9 @@ namespace Avalonia.Controls
 
             if (text is null) return;
 
-            _undoRedoHelper.Snapshot();
+            SnapshotUndoRedo();
             HandleTextInput(text);
-            _undoRedoHelper.Snapshot();
+            SnapshotUndoRedo();
         }
 
         protected override void OnKeyDown(KeyEventArgs e)
@@ -638,7 +692,7 @@ namespace Avalonia.Controls
                 Paste();
                 handled = true;
             }
-            else if (Match(keymap.Undo))
+            else if (Match(keymap.Undo) && IsUndoEnabled)
             {
                 try
                 {
@@ -652,7 +706,7 @@ namespace Avalonia.Controls
 
                 handled = true;
             }
-            else if (Match(keymap.Redo))
+            else if (Match(keymap.Redo) && IsUndoEnabled)
             {
                 try
                 {
@@ -752,7 +806,7 @@ namespace Avalonia.Controls
                         break;
 
                     case Key.Back:
-                        _undoRedoHelper.Snapshot();
+                        SnapshotUndoRedo();
                         if (hasWholeWordModifiers && SelectionStart == SelectionEnd)
                         {
                             SetSelectionForControlBackspace();
@@ -776,13 +830,13 @@ namespace Avalonia.Controls
                             CaretIndex -= removedCharacters;
                             ClearSelection();
                         }
-                        _undoRedoHelper.Snapshot();
+                        SnapshotUndoRedo();
 
                         handled = true;
                         break;
 
                     case Key.Delete:
-                        _undoRedoHelper.Snapshot();
+                        SnapshotUndoRedo();
                         if (hasWholeWordModifiers && SelectionStart == SelectionEnd)
                         {
                             SetSelectionForControlDelete();
@@ -804,7 +858,7 @@ namespace Avalonia.Controls
                             SetTextInternal(text.Substring(0, caretIndex) +
                                             text.Substring(caretIndex + removedCharacters));
                         }
-                        _undoRedoHelper.Snapshot();
+                        SnapshotUndoRedo();
 
                         handled = true;
                         break;
@@ -812,9 +866,9 @@ namespace Avalonia.Controls
                     case Key.Enter:
                         if (AcceptsReturn)
                         {
-                            _undoRedoHelper.Snapshot();
+                            SnapshotUndoRedo();
                             HandleTextInput(NewLine);
-                            _undoRedoHelper.Snapshot();
+                            SnapshotUndoRedo();
                             handled = true;
                         }
 
@@ -823,9 +877,9 @@ namespace Avalonia.Controls
                     case Key.Tab:
                         if (AcceptsTab)
                         {
-                            _undoRedoHelper.Snapshot();
+                            SnapshotUndoRedo();
                             HandleTextInput("\t");
-                            _undoRedoHelper.Snapshot();
+                            SnapshotUndoRedo();
                             handled = true;
                         }
                         else
@@ -1236,7 +1290,7 @@ namespace Avalonia.Controls
 
         private void UpdatePseudoclasses()
         {
-            PseudoClasses.Set(":empty", string.IsNullOrWhiteSpace(Text));
+            PseudoClasses.Set(":empty", string.IsNullOrEmpty(Text));
         }
 
         private bool IsPasswordBox => PasswordChar != default(char);
@@ -1251,5 +1305,13 @@ namespace Avalonia.Controls
                 ClearSelection();
             }
         }
+
+        private void SnapshotUndoRedo()
+        {
+            if (IsUndoEnabled)
+            {
+                _undoRedoHelper.Snapshot();
+            }
+        }
     }
 }

src/Avalonia.Controls/TickBar.cs

@@ -1,6 +1,7 @@
 using Avalonia.Collections;
 using Avalonia.Layout;
 using Avalonia.Media;
+using Avalonia.Media.Immutable;
 using Avalonia.Utilities;
 
 namespace Avalonia.Controls
@@ -295,7 +296,7 @@ namespace Avalonia.Controls
                 endPoint = pt;
             }
 
-            var pen = new Pen(Fill, 1.0d);
+            var pen = new ImmutablePen(Fill?.ToImmutable(), 1.0d);
 
             // Is it Vertical?
             if (Placement == TickBarPlacement.Left || Placement == TickBarPlacement.Right)

src/Avalonia.Controls/TopLevel.cs

@@ -42,6 +42,12 @@ namespace Avalonia.Controls
         public static readonly DirectProperty<TopLevel, Size> ClientSizeProperty =
             AvaloniaProperty.RegisterDirect<TopLevel, Size>(nameof(ClientSize), o => o.ClientSize);
 
+        /// <summary>
+        /// Defines the <see cref="FrameSize"/> property.
+        /// </summary>
+        public static readonly DirectProperty<TopLevel, Size?> FrameSizeProperty =
+            AvaloniaProperty.RegisterDirect<TopLevel, Size?>(nameof(FrameSize), o => o.FrameSize);
+
         /// <summary>
         /// Defines the <see cref="IInputRoot.PointerOverElement"/> property.
         /// </summary>
@@ -74,6 +80,7 @@ namespace Avalonia.Controls
         private readonly IPlatformRenderInterface _renderInterface;
         private readonly IGlobalStyles _globalStyles;
         private Size _clientSize;
+        private Size? _frameSize;
         private WindowTransparencyLevel _actualTransparencyLevel;
         private ILayoutManager _layoutManager;
         private Border _transparencyFallbackBorder;
@@ -161,6 +168,7 @@ namespace Avalonia.Controls
             styler?.ApplyStyles(this);
 
             ClientSize = impl.ClientSize;
+            FrameSize = impl.FrameSize;
             
             this.GetObservable(PointerOverElementProperty)
                 .Select(
@@ -197,6 +205,15 @@ namespace Avalonia.Controls
             protected set { SetAndRaise(ClientSizeProperty, ref _clientSize, value); }
         }
 
+        /// <summary>
+        /// Gets or sets the total size of the window.
+        /// </summary>
+        public Size? FrameSize
+        {
+            get { return _frameSize; }
+            protected set { SetAndRaise(FrameSizeProperty, ref _frameSize, value); }
+        }
+
         /// <summary>
         /// Gets or sets the <see cref="WindowTransparencyLevel"/> that the TopLevel should use when possible.
         /// </summary>
@@ -366,6 +383,7 @@ namespace Avalonia.Controls
         protected virtual void HandleResized(Size clientSize)
         {
             ClientSize = clientSize;
+            FrameSize = PlatformImpl.FrameSize;
             Width = clientSize.Width;
             Height = clientSize.Height;
             LayoutManager.ExecuteLayoutPass();

src/Avalonia.Controls/TreeView.cs

@@ -166,11 +166,9 @@ namespace Avalonia.Controls
         {
             item.IsExpanded = true;
 
-            var panel = item.Presenter.Panel;
-
-            if (panel != null)
+            if (item.Presenter?.Panel != null)
             {
-                foreach (var child in panel.Children)
+                foreach (var child in item.Presenter.Panel.Children)
                 {
                     if (child is TreeViewItem treeViewItem)
                     {

src/Avalonia.Controls/Utils/BorderRenderHelper.cs

@@ -1,5 +1,6 @@
 using System;
 using Avalonia.Media;
+using Avalonia.Media.Immutable;
 using Avalonia.Platform;
 using Avalonia.Utilities;
 
@@ -114,9 +115,9 @@ namespace Avalonia.Controls.Utils
                 var borderThickness = _borderThickness.Top;
                 IPen pen = null;
 
-                if (borderThickness > 0)
+                if (borderBrush != null && borderThickness > 0)
                 {
-                    pen = new Pen(borderBrush, borderThickness);
+                    pen = new ImmutablePen(borderBrush.ToImmutable(), borderThickness);
                 }
 
                 var rect = new Rect(_size);

src/Avalonia.Controls/Utils/UndoRedoHelper.cs

@@ -22,6 +22,10 @@ namespace Avalonia.Controls.Utils
 
         private LinkedListNode<TState> _currentNode;
 
+        /// <summary>
+        /// Maximum number of states this helper can store for undo/redo.
+        /// If -1, no limit is imposed.
+        /// </summary>
         public int Limit { get; set; } = 10;
 
         public UndoRedoHelper(IUndoRedoHost host)
@@ -54,7 +58,10 @@ namespace Avalonia.Controls.Utils
         public bool HasState => _currentNode != null;
         public void UpdateLastState(TState state)
         {
-            _states.Last.Value = state;
+            if (_states.Last != null)
+            {
+                _states.Last.Value = state;
+            } 
         }
 
         public void UpdateLastState()
@@ -86,7 +93,7 @@ namespace Avalonia.Controls.Utils
                     DiscardRedo();
                 _states.AddLast(current);
                 _currentNode = _states.Last;
-                if (_states.Count > Limit)
+                if (Limit != -1 && _states.Count > Limit)
                     _states.RemoveFirst();
             }
         }

src/Avalonia.Controls/Window.cs

@@ -592,6 +592,14 @@ namespace Avalonia.Controls
                     owner.RemoveChild(this);
                 }
 
+                if (_children.Count > 0)
+                {
+                    foreach (var child in _children.ToArray())
+                    {
+                        child.child.Hide();
+                    }
+                }
+
                 Owner = null;
 
                 PlatformImpl?.Hide();
@@ -635,6 +643,22 @@ namespace Avalonia.Controls
                 throw new InvalidOperationException("Cannot re-show a closed window.");
             }
 
+            if (parent != null)
+            {
+                if (parent.PlatformImpl == null)
+                {
+                    throw new InvalidOperationException("Cannot show a window with a closed parent.");
+                }
+                else if (parent == this)
+                {
+                    throw new InvalidOperationException("A Window cannot be its own parent.");
+                }
+                else if (!parent.IsVisible)
+                {
+                    throw new InvalidOperationException("Cannot show window with non-visible parent.");
+                }
+            }
+
             if (IsVisible)
             {
                 return;
@@ -708,11 +732,22 @@ namespace Avalonia.Controls
             {
                 throw new ArgumentNullException(nameof(owner));
             }
-
-            if (IsVisible)
+            else if (owner.PlatformImpl == null)
+            {
+                throw new InvalidOperationException("Cannot show a window with a closed owner.");
+            }
+            else if (owner == this)
+            {
+                throw new InvalidOperationException("A Window cannot be its own owner.");
+            }
+            else if (IsVisible)
             {
                 throw new InvalidOperationException("The window is already being shown.");
             }
+            else if (!owner.IsVisible)
+            {
+                throw new InvalidOperationException("Cannot show window with non-visible parent.");
+            }
 
             RaiseEvent(new RoutedEventArgs(WindowOpenedEvent));
 

src/Avalonia.Controls/WindowBase.cs

@@ -222,6 +222,7 @@ namespace Avalonia.Controls
         protected override void HandleResized(Size clientSize)
         {
             ClientSize = clientSize;
+            FrameSize = PlatformImpl.FrameSize;
             LayoutManager.ExecuteLayoutPass();
             Renderer?.Resized(clientSize);
         }

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -21,6 +21,7 @@ namespace Avalonia.DesignerSupport.Remote
         public IPlatformHandle Handle { get; }
         public Size MaxAutoSizeHint { get; }
         public Size ClientSize { get; }
+        public Size? FrameSize => null;
         public double RenderScaling { get; } = 1.0;
         public double DesktopScaling => 1.0;
         public IEnumerable<object> Surfaces { get; }

src/Avalonia.Diagnostics/Avalonia.Diagnostics.csproj

@@ -3,12 +3,16 @@
     <TargetFramework>netstandard2.0</TargetFramework>
     <RootNamespace>Avalonia</RootNamespace>
     <PackageId>Avalonia.Diagnostics</PackageId>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Update="**\*.xaml.cs">
       <DependentUpon>%(Filename)</DependentUpon>
     </Compile>
   </ItemGroup>
+  <ItemGroup>
+    <Compile Include="..\Avalonia.Base\Metadata\NullableAttributes.cs" Link="NullableAttributes.cs" />
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\Avalonia.Controls.DataGrid\Avalonia.Controls.DataGrid.csproj" />
     <ProjectReference Include="..\Markup\Avalonia.Markup.Xaml\Avalonia.Markup.Xaml.csproj" />

src/Avalonia.Diagnostics/Diagnostics/Controls/FilterTextBox.axaml

@@ -27,7 +27,9 @@
             <Image>
               <DrawingImage>
                 <GeometryDrawing Brush="{Binding $parent[ToggleButton].Foreground}">
-                  M7.495 9.052L8.386 11.402H9.477L6.237 3H5.217L2 11.402H3.095L3.933 9.052H7.495ZM5.811 4.453L5.855 4.588L7.173 8.162H4.255L5.562 4.588L5.606 4.453L5.644 4.297L5.676 4.145L5.697 4.019H5.72L5.744 4.145L5.773 4.297L5.811 4.453ZM13.795 10.464V11.4H14.755V7.498C14.755 6.779 14.575 6.226 14.216 5.837C13.857 5.448 13.327 5.254 12.628 5.254C12.429 5.254 12.227 5.273 12.022 5.31C11.817 5.347 11.622 5.394 11.439 5.451C11.256 5.508 11.091 5.569 10.944 5.636C10.797 5.703 10.683 5.765 10.601 5.824V6.808C10.867 6.578 11.167 6.397 11.505 6.268C11.843 6.139 12.194 6.075 12.557 6.075C12.745 6.075 12.915 6.103 13.07 6.16C13.225 6.217 13.357 6.306 13.466 6.427C13.575 6.548 13.659 6.706 13.718 6.899C13.777 7.092 13.806 7.326 13.806 7.599L11.995 7.851C11.651 7.898 11.355 7.977 11.107 8.088C10.859 8.199 10.654 8.339 10.492 8.507C10.33 8.675 10.21 8.868 10.132 9.087C10.054 9.306 10.015 9.546 10.015 9.808C10.015 10.054 10.057 10.283 10.139 10.496C10.221 10.709 10.342 10.893 10.502 11.047C10.662 11.201 10.862 11.323 11.1 11.413C11.338 11.503 11.613 11.548 11.926 11.548C12.328 11.548 12.686 11.456 13.001 11.27C13.316 11.084 13.573 10.816 13.772 10.464H13.795ZM11.667 8.721C11.843 8.657 12.068 8.607 12.341 8.572L13.806 8.367V8.976C13.806 9.222 13.765 9.451 13.683 9.664C13.601 9.877 13.486 10.063 13.34 10.221C13.194 10.379 13.019 10.503 12.816 10.593C12.613 10.683 12.39 10.728 12.148 10.728C11.961 10.728 11.795 10.703 11.653 10.652C11.511 10.601 11.392 10.53 11.296 10.441C11.2 10.352 11.127 10.247 11.076 10.125C11.025 10.003 11 9.873 11 9.732C11 9.568 11.018 9.421 11.055 9.292C11.092 9.163 11.16 9.051 11.257 8.958C11.354 8.865 11.491 8.785 11.667 8.721Z
+                    <GeometryDrawing.Geometry>
+                    M7.495 9.052L8.386 11.402H9.477L6.237 3H5.217L2 11.402H3.095L3.933 9.052H7.495ZM5.811 4.453L5.855 4.588L7.173 8.162H4.255L5.562 4.588L5.606 4.453L5.644 4.297L5.676 4.145L5.697 4.019H5.72L5.744 4.145L5.773 4.297L5.811 4.453ZM13.795 10.464V11.4H14.755V7.498C14.755 6.779 14.575 6.226 14.216 5.837C13.857 5.448 13.327 5.254 12.628 5.254C12.429 5.254 12.227 5.273 12.022 5.31C11.817 5.347 11.622 5.394 11.439 5.451C11.256 5.508 11.091 5.569 10.944 5.636C10.797 5.703 10.683 5.765 10.601 5.824V6.808C10.867 6.578 11.167 6.397 11.505 6.268C11.843 6.139 12.194 6.075 12.557 6.075C12.745 6.075 12.915 6.103 13.07 6.16C13.225 6.217 13.357 6.306 13.466 6.427C13.575 6.548 13.659 6.706 13.718 6.899C13.777 7.092 13.806 7.326 13.806 7.599L11.995 7.851C11.651 7.898 11.355 7.977 11.107 8.088C10.859 8.199 10.654 8.339 10.492 8.507C10.33 8.675 10.21 8.868 10.132 9.087C10.054 9.306 10.015 9.546 10.015 9.808C10.015 10.054 10.057 10.283 10.139 10.496C10.221 10.709 10.342 10.893 10.502 11.047C10.662 11.201 10.862 11.323 11.1 11.413C11.338 11.503 11.613 11.548 11.926 11.548C12.328 11.548 12.686 11.456 13.001 11.27C13.316 11.084 13.573 10.816 13.772 10.464H13.795ZM11.667 8.721C11.843 8.657 12.068 8.607 12.341 8.572L13.806 8.367V8.976C13.806 9.222 13.765 9.451 13.683 9.664C13.601 9.877 13.486 10.063 13.34 10.221C13.194 10.379 13.019 10.503 12.816 10.593C12.613 10.683 12.39 10.728 12.148 10.728C11.961 10.728 11.795 10.703 11.653 10.652C11.511 10.601 11.392 10.53 11.296 10.441C11.2 10.352 11.127 10.247 11.076 10.125C11.025 10.003 11 9.873 11 9.732C11 9.568 11.018 9.421 11.055 9.292C11.092 9.163 11.16 9.051 11.257 8.958C11.354 8.865 11.491 8.785 11.667 8.721Z
+                    </GeometryDrawing.Geometry>
                 </GeometryDrawing>
               </DrawingImage>
             </Image>
@@ -38,7 +40,9 @@
             <Image>
               <DrawingImage>
                 <GeometryDrawing Brush="{Binding $parent[ToggleButton].Foreground}">
-                  M1 2H15V3H1V2ZM14 4H13V12H14V4ZM11.272 8.387C11.194 8.088 11.073 7.825 10.912 7.601C10.751 7.377 10.547 7.2 10.303 7.071C10.059 6.942 9.769 6.878 9.437 6.878C9.239 6.878 9.057 6.902 8.89 6.951C8.725 7 8.574 7.068 8.437 7.156C8.301 7.244 8.18 7.35 8.072 7.474L7.893 7.732V4.578H7V12H7.893V11.425L8.019 11.6C8.106 11.702 8.208 11.79 8.323 11.869C8.44 11.947 8.572 12.009 8.721 12.055C8.87 12.101 9.035 12.123 9.219 12.123C9.572 12.123 9.885 12.052 10.156 11.911C10.428 11.768 10.655 11.573 10.838 11.325C11.021 11.075 11.159 10.782 11.252 10.446C11.345 10.108 11.392 9.743 11.392 9.349C11.391 9.007 11.352 8.686 11.272 8.387ZM9.793 7.78C9.944 7.851 10.075 7.956 10.183 8.094C10.292 8.234 10.377 8.407 10.438 8.611C10.489 8.785 10.52 8.982 10.527 9.198L10.52 9.323C10.52 9.65 10.487 9.943 10.42 10.192C10.353 10.438 10.259 10.645 10.142 10.806C10.025 10.968 9.882 11.091 9.721 11.172C9.399 11.334 8.961 11.338 8.652 11.187C8.499 11.112 8.366 11.012 8.259 10.891C8.174 10.795 8.103 10.675 8.041 10.524C8.041 10.524 7.862 10.077 7.862 9.577C7.862 9.077 8.041 8.575 8.041 8.575C8.103 8.398 8.177 8.257 8.265 8.145C8.379 8.002 8.521 7.886 8.689 7.8C8.857 7.714 9.054 7.671 9.276 7.671C9.466 7.671 9.64 7.708 9.793 7.78ZM15 13H1V14H15V13ZM2.813 10L2.085 12.031H1L1.025 11.959L3.466 4.87305H4.407L6.892 12.031H5.81L5.032 10H2.813ZM3.934 6.42205H3.912L3.007 9.17505H4.848L3.934 6.42205Z
+                    <GeometryDrawing.Geometry>
+                    M1 2H15V3H1V2ZM14 4H13V12H14V4ZM11.272 8.387C11.194 8.088 11.073 7.825 10.912 7.601C10.751 7.377 10.547 7.2 10.303 7.071C10.059 6.942 9.769 6.878 9.437 6.878C9.239 6.878 9.057 6.902 8.89 6.951C8.725 7 8.574 7.068 8.437 7.156C8.301 7.244 8.18 7.35 8.072 7.474L7.893 7.732V4.578H7V12H7.893V11.425L8.019 11.6C8.106 11.702 8.208 11.79 8.323 11.869C8.44 11.947 8.572 12.009 8.721 12.055C8.87 12.101 9.035 12.123 9.219 12.123C9.572 12.123 9.885 12.052 10.156 11.911C10.428 11.768 10.655 11.573 10.838 11.325C11.021 11.075 11.159 10.782 11.252 10.446C11.345 10.108 11.392 9.743 11.392 9.349C11.391 9.007 11.352 8.686 11.272 8.387ZM9.793 7.78C9.944 7.851 10.075 7.956 10.183 8.094C10.292 8.234 10.377 8.407 10.438 8.611C10.489 8.785 10.52 8.982 10.527 9.198L10.52 9.323C10.52 9.65 10.487 9.943 10.42 10.192C10.353 10.438 10.259 10.645 10.142 10.806C10.025 10.968 9.882 11.091 9.721 11.172C9.399 11.334 8.961 11.338 8.652 11.187C8.499 11.112 8.366 11.012 8.259 10.891C8.174 10.795 8.103 10.675 8.041 10.524C8.041 10.524 7.862 10.077 7.862 9.577C7.862 9.077 8.041 8.575 8.041 8.575C8.103 8.398 8.177 8.257 8.265 8.145C8.379 8.002 8.521 7.886 8.689 7.8C8.857 7.714 9.054 7.671 9.276 7.671C9.466 7.671 9.64 7.708 9.793 7.78ZM15 13H1V14H15V13ZM2.813 10L2.085 12.031H1L1.025 11.959L3.466 4.87305H4.407L6.892 12.031H5.81L5.032 10H2.813ZM3.934 6.42205H3.912L3.007 9.17505H4.848L3.934 6.42205Z
+                    </GeometryDrawing.Geometry>
                 </GeometryDrawing>
               </DrawingImage>
             </Image>
@@ -49,7 +53,9 @@
             <Image>
               <DrawingImage>
                 <GeometryDrawing Brush="{Binding $parent[ToggleButton].Foreground}">
-                  M10.0122 2H10.9879V5.11346L13.5489 3.55609L14.034 4.44095L11.4702 6L14.034 7.55905L13.5489 8.44391L10.9879 6.88654V10H10.0122V6.88654L7.45114 8.44391L6.96606 7.55905L9.5299 6L6.96606 4.44095L7.45114 3.55609L10.0122 5.11346V2ZM2 10H6V14H2V10Z
+                    <GeometryDrawing.Geometry>
+                    M10.0122 2H10.9879V5.11346L13.5489 3.55609L14.034 4.44095L11.4702 6L14.034 7.55905L13.5489 8.44391L10.9879 6.88654V10H10.0122V6.88654L7.45114 8.44391L6.96606 7.55905L9.5299 6L6.96606 4.44095L7.45114 3.55609L10.0122 5.11346V2ZM2 10H6V14H2V10Z
+                    </GeometryDrawing.Geometry>
                 </GeometryDrawing>
               </DrawingImage>
             </Image>

src/Avalonia.Diagnostics/Diagnostics/Controls/ThicknessEditor.cs

@@ -10,8 +10,8 @@ namespace Avalonia.Diagnostics.Controls
             AvaloniaProperty.RegisterDirect<ThicknessEditor, Thickness>(nameof(Thickness), o => o.Thickness,
                 (o, v) => o.Thickness = v, defaultBindingMode: BindingMode.TwoWay);
 
-        public static readonly DirectProperty<ThicknessEditor, string> HeaderProperty =
-            AvaloniaProperty.RegisterDirect<ThicknessEditor, string>(nameof(Header), o => o.Header,
+        public static readonly DirectProperty<ThicknessEditor, string?> HeaderProperty =
+            AvaloniaProperty.RegisterDirect<ThicknessEditor, string?>(nameof(Header), o => o.Header,
                 (o, v) => o.Header = v);
 
         public static readonly DirectProperty<ThicknessEditor, bool> IsPresentProperty =
@@ -36,7 +36,7 @@ namespace Avalonia.Diagnostics.Controls
             AvaloniaProperty.Register<ThicknessEditor, IBrush>(nameof(Highlight));
 
         private Thickness _thickness;
-        private string _header;
+        private string? _header;
         private bool _isPresent = true;
         private double _left;
         private double _top;
@@ -50,7 +50,7 @@ namespace Avalonia.Diagnostics.Controls
             set => SetAndRaise(ThicknessProperty, ref _thickness, value);
         }
 
-        public string Header
+        public string? Header
         {
             get => _header;
             set => SetAndRaise(HeaderProperty, ref _header, value);

src/Avalonia.Diagnostics/Diagnostics/Converters/BoolToOpacityConverter.cs

@@ -8,12 +8,17 @@ namespace Avalonia.Diagnostics.Converters
     {
         public double Opacity { get; set; }
 
-        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
         {
-            return (bool)value ? 1d : Opacity;
+            if (value is bool boolean && boolean)
+            {
+                return 1d;
+            }
+
+            return Opacity;
         }
 
-        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
         {
             throw new NotImplementedException();
         }

src/Avalonia.Diagnostics/Diagnostics/Converters/EnumToCheckedConverter.cs

@@ -7,12 +7,12 @@ namespace Avalonia.Diagnostics.Converters
 {
     internal class EnumToCheckedConverter : IValueConverter
     {
-        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
         {
             return Equals(value, parameter);
         }
 
-        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
         {
             if (value is bool isChecked && isChecked)
             {

src/Avalonia.Diagnostics/Diagnostics/DevTools.cs

@@ -6,13 +6,12 @@ using Avalonia.Diagnostics.Views;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 
-#nullable enable 
-
 namespace Avalonia.Diagnostics
 {
     public static class DevTools
     {
-        private static readonly Dictionary<TopLevel, Window> s_open = new Dictionary<TopLevel, Window>();
+        private static readonly Dictionary<TopLevel, MainWindow> s_open =
+            new Dictionary<TopLevel, MainWindow>();
 
         public static IDisposable Attach(TopLevel root, KeyGesture gesture)
         {
@@ -24,7 +23,7 @@ namespace Avalonia.Diagnostics
 
         public static IDisposable Attach(TopLevel root, DevToolsOptions options)
         {
-            void PreviewKeyDown(object sender, KeyEventArgs e)
+            void PreviewKeyDown(object? sender, KeyEventArgs e)
             {
                 if (options.Gesture.Matches(e))
                 {
@@ -54,6 +53,7 @@ namespace Avalonia.Diagnostics
                     Width = options.Size.Width,
                     Height = options.Size.Height,
                 };
+                window.SetOptions(options);
 
                 window.Closed += DevToolsClosed;
                 s_open.Add(root, window);
@@ -71,10 +71,10 @@ namespace Avalonia.Diagnostics
             return Disposable.Create(() => window?.Close());
         }
 
-        private static void DevToolsClosed(object sender, EventArgs e)
+        private static void DevToolsClosed(object? sender, EventArgs e)
         {
-            var window = (MainWindow)sender;
-            s_open.Remove(window.Root);
+            var window = (MainWindow)sender!;
+            s_open.Remove(window.Root!);
             window.Closed -= DevToolsClosed;
         }
     }

src/Avalonia.Diagnostics/Diagnostics/DevToolsOptions.cs

@@ -22,5 +22,10 @@ namespace Avalonia.Diagnostics
         /// Gets or sets the initial size of the DevTools window. The default value is 1280x720.
         /// </summary>
         public Size Size { get; set; } = new Size(1280, 720);
+
+        /// <summary>
+        /// Get or set the startup screen index where the DevTools window will be displayed.
+        /// </summary>
+        public int? StartupScreenIndex { get; set; }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/Models/ConsoleContext.cs

@@ -22,8 +22,8 @@ The following commands are available:
 clear(): Clear the output history
 ";
 
-        public dynamic e { get; internal set; }
-        public dynamic root { get; internal set; }
+        public dynamic? e { get; internal set; }
+        public dynamic? root { get; internal set; }
 
         internal static object NoOutput { get; } = new object();
 

src/Avalonia.Diagnostics/Diagnostics/Models/EventChainLink.cs

@@ -7,9 +7,7 @@ namespace Avalonia.Diagnostics.Models
     {
         public EventChainLink(object handler, bool handled, RoutingStrategies route)
         {
-            Contract.Requires<ArgumentNullException>(handler != null);
-
-            Handler = handler;
+            Handler = handler ?? throw new ArgumentNullException(nameof(handler));
             Handled = handled;
             Route = route;
         }

src/Avalonia.Diagnostics/Diagnostics/ViewLocator.cs

@@ -9,12 +9,12 @@ namespace Avalonia.Diagnostics
     {
         public IControl Build(object data)
         {
-            var name = data.GetType().FullName.Replace("ViewModel", "View");
+            var name = data.GetType().FullName!.Replace("ViewModel", "View");
             var type = Type.GetType(name);
 
             if (type != null)
             {
-                return (Control)Activator.CreateInstance(type);
+                return (Control)Activator.CreateInstance(type)!;
             }
             else
             {

src/Avalonia.Diagnostics/Diagnostics/ViewModels/AvaloniaPropertyViewModel.cs

@@ -1,17 +1,17 @@
-using System.ComponentModel;
-using Avalonia.Collections;
-
 namespace Avalonia.Diagnostics.ViewModels
 {
     internal class AvaloniaPropertyViewModel : PropertyViewModel
     {
         private readonly AvaloniaObject _target;
         private string _type;
-        private object _value;
+        private object? _value;
         private string _priority;
         private string _group;
 
+#nullable disable
+        // Remove "nullable disable" after MemberNotNull will work on our CI.
         public AvaloniaPropertyViewModel(AvaloniaObject o, AvaloniaProperty property)
+#nullable restore
         {
             _target = o;
             Property = property;
@@ -20,25 +20,17 @@ namespace Avalonia.Diagnostics.ViewModels
                 $"[{property.OwnerType.Name}.{property.Name}]" :
                 property.Name;
 
-            if (property.IsDirect)
-            {
-                _group = "Properties";
-                Priority = "Direct";
-            }
-
             Update();
         }
 
         public AvaloniaProperty Property { get; }
         public override object Key => Property;
         public override string Name { get; }
-        public bool IsAttached => Property.IsAttached;
+        public override bool? IsAttached => 
+            Property.IsAttached;
 
-        public string Priority
-        {
-            get => _priority;
-            private set => RaiseAndSetIfChanged(ref _priority, value);
-        }
+        public override string Priority =>
+            _priority;
 
         public override string Type => _type;
 
@@ -56,40 +48,37 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public override string Group
-        {
-            get => _group;
-        }
+        public override string Group => _group;
 
+        // [MemberNotNull(nameof(_type), nameof(_group), nameof(_priority))]
         public override void Update()
         {
             if (Property.IsDirect)
             {
                 RaiseAndSetIfChanged(ref _value, _target.GetValue(Property), nameof(Value));
-                RaiseAndSetIfChanged(ref _type, _value?.GetType().Name, nameof(Type));
+                RaiseAndSetIfChanged(ref _type, _value?.GetType().Name ?? Property.PropertyType.Name, nameof(Type));
+                RaiseAndSetIfChanged(ref _priority, "Direct", nameof(Priority));
+
+                _group = "Properties";
             }
             else
             {
                 var val = _target.GetDiagnostic(Property);
 
                 RaiseAndSetIfChanged(ref _value, val?.Value, nameof(Value));
-                RaiseAndSetIfChanged(ref _type, _value?.GetType().Name, nameof(Type));
+                RaiseAndSetIfChanged(ref _type, _value?.GetType().Name ?? Property.PropertyType.Name, nameof(Type));
 
                 if (val != null)
                 {
-                    SetGroup(IsAttached ? "Attached Properties" : "Properties");
-                    Priority = val.Priority.ToString();
+                    RaiseAndSetIfChanged(ref _priority, val.Priority.ToString(), nameof(Priority));
+                    RaiseAndSetIfChanged(ref _group, IsAttached == true ? "Attached Properties" : "Properties", nameof(Group));
                 }
                 else
                 {
-                    SetGroup(Priority = "Unset");
+                    RaiseAndSetIfChanged(ref _priority, "Unset", nameof(Priority));
+                    RaiseAndSetIfChanged(ref _group, "Unset", nameof(Group));
                 }
             }
         }
-
-        private void SetGroup(string group)
-        {
-            RaiseAndSetIfChanged(ref _group, group, nameof(Group));
-        }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/ClrPropertyViewModel.cs

@@ -1,5 +1,4 @@
-using System.ComponentModel;
-using System.Reflection;
+using System.Reflection;
 
 namespace Avalonia.Diagnostics.ViewModels
 {
@@ -7,14 +6,17 @@ namespace Avalonia.Diagnostics.ViewModels
     {
         private readonly object _target;
         private string _type;
-        private object _value;
+        private object? _value;
 
+#nullable disable
+        // Remove "nullable disable" after MemberNotNull will work on our CI.
         public ClrPropertyViewModel(object o, PropertyInfo property)
+#nullable restore
         {
             _target = o;
             Property = property;
 
-            if (!property.DeclaringType.IsInterface)
+            if (property.DeclaringType == null || !property.DeclaringType.IsInterface)
             {
                 Name = property.Name;
             }
@@ -47,11 +49,18 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
+        public override string Priority => 
+            string.Empty;
+
+        public override bool? IsAttached => 
+            default;
+
+        // [MemberNotNull(nameof(_type))]
         public override void Update()
         {
             var val = Property.GetValue(_target);
             RaiseAndSetIfChanged(ref _value, val, nameof(Value));
-            RaiseAndSetIfChanged(ref _type, _value?.GetType().Name, nameof(Type));
+            RaiseAndSetIfChanged(ref _type, _value?.GetType().Name ?? Property.PropertyType.Name, nameof(Type));
         }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/ConsoleViewModel.cs

@@ -15,11 +15,12 @@ namespace Avalonia.Diagnostics.ViewModels
         private int _historyIndex = -1;
         private string _input;
         private bool _isVisible;
-        private ScriptState<object> _state;
+        private ScriptState<object>? _state;
 
         public ConsoleViewModel(Action<ConsoleContext> updateContext)
         {
             _context = new ConsoleContext(this);
+            _input = string.Empty;
             _updateContext = updateContext;
         }
 

src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlDetailsViewModel.cs

@@ -18,10 +18,10 @@ namespace Avalonia.Diagnostics.ViewModels
     {
         private readonly IVisual _control;
         private readonly IDictionary<object, List<PropertyViewModel>> _propertyIndex;
-        private AvaloniaPropertyViewModel _selectedProperty;
+        private PropertyViewModel? _selectedProperty;
         private bool _snapshotStyles;
         private bool _showInactiveStyles;
-        private string _styleStatus;
+        private string? _styleStatus;
 
         public ControlDetailsViewModel(TreePageViewModel treePage, IVisual control)
         {
@@ -83,7 +83,8 @@ namespace Avalonia.Diagnostics.ViewModels
                     {
                         foreach (var setter in style.Setters)
                         {
-                            if (setter is Setter regularSetter)
+                            if (setter is Setter regularSetter
+                                && regularSetter.Property != null)
                             {
                                 var setterValue = regularSetter.Value;
 
@@ -115,13 +116,14 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        private (object resourceKey, bool isDynamic)? GetResourceInfo(object value)
+        private (object resourceKey, bool isDynamic)? GetResourceInfo(object? value)
         {
             if (value is StaticResourceExtension staticResource)
             {
                 return (staticResource.ResourceKey, false);
             }
-            else if (value is DynamicResourceExtension dynamicResource)
+            else if (value is DynamicResourceExtension dynamicResource
+                && dynamicResource.ResourceKey != null)
             {
                 return (dynamicResource.ResourceKey, true);
             }
@@ -137,7 +139,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public ObservableCollection<PseudoClassViewModel> PseudoClasses { get; }
 
-        public AvaloniaPropertyViewModel SelectedProperty
+        public PropertyViewModel? SelectedProperty
         {
             get => _selectedProperty;
             set => RaiseAndSetIfChanged(ref _selectedProperty, value);
@@ -155,7 +157,7 @@ namespace Avalonia.Diagnostics.ViewModels
             set => RaiseAndSetIfChanged(ref _showInactiveStyles, value);
         }
 
-        public string StyleStatus
+        public string? StyleStatus
         {
             get => _styleStatus;
             set => RaiseAndSetIfChanged(ref _styleStatus, value);
@@ -248,7 +250,7 @@ namespace Avalonia.Diagnostics.ViewModels
                 .Select(x => new ClrPropertyViewModel(o, x));
         }
 
-        private void ControlPropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        private void ControlPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             if (_propertyIndex.TryGetValue(e.Property, out var properties))
             {
@@ -261,9 +263,10 @@ namespace Avalonia.Diagnostics.ViewModels
             Layout.ControlPropertyChanged(sender, e);
         }
 
-        private void ControlPropertyChanged(object sender, PropertyChangedEventArgs e)
+        private void ControlPropertyChanged(object? sender, PropertyChangedEventArgs e)
         {
-            if (_propertyIndex.TryGetValue(e.PropertyName, out var properties))
+            if (e.PropertyName != null
+                && _propertyIndex.TryGetValue(e.PropertyName, out var properties))
             {
                 foreach (var property in properties)
                 {
@@ -277,7 +280,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        private void OnClassesChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void OnClassesChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (!SnapshotStyles)
             {
@@ -349,10 +352,10 @@ namespace Avalonia.Diagnostics.ViewModels
         {
             public static PropertyComparer Instance { get; } = new PropertyComparer();
 
-            public int Compare(PropertyViewModel x, PropertyViewModel y)
+            public int Compare(PropertyViewModel? x, PropertyViewModel? y)
             {
-                var groupX = GroupIndex(x.Group);
-                var groupY = GroupIndex(y.Group);
+                var groupX = GroupIndex(x?.Group);
+                var groupY = GroupIndex(y?.Group);
 
                 if (groupX != groupY)
                 {
@@ -360,11 +363,11 @@ namespace Avalonia.Diagnostics.ViewModels
                 }
                 else
                 {
-                    return string.CompareOrdinal(x.Name, y.Name);
+                    return string.CompareOrdinal(x?.Name, y?.Name);
                 }
             }
 
-            private int GroupIndex(string group)
+            private int GroupIndex(string? group)
             {
                 switch (group)
                 {

src/Avalonia.Diagnostics/Diagnostics/ViewModels/ControlLayoutViewModel.cs

@@ -12,14 +12,14 @@ namespace Avalonia.Diagnostics.ViewModels
         private readonly IVisual _control;
         private Thickness _borderThickness;
         private double _height;
-        private string _heightConstraint;
+        private string? _heightConstraint;
         private HorizontalAlignment _horizontalAlignment;
         private Thickness _marginThickness;
         private Thickness _paddingThickness;
         private bool _updatingFromControl;
         private VerticalAlignment _verticalAlignment;
         private double _width;
-        private string _widthConstraint;
+        private string? _widthConstraint;
 
         public ControlLayoutViewModel(IVisual control)
         {
@@ -80,13 +80,13 @@ namespace Avalonia.Diagnostics.ViewModels
             private set => RaiseAndSetIfChanged(ref _height, value);
         }
 
-        public string WidthConstraint
+        public string? WidthConstraint
         {
             get => _widthConstraint;
             private set => RaiseAndSetIfChanged(ref _widthConstraint, value);
         }
 
-        public string HeightConstraint
+        public string? HeightConstraint
         {
             get => _heightConstraint;
             private set => RaiseAndSetIfChanged(ref _heightConstraint, value);
@@ -112,7 +112,7 @@ namespace Avalonia.Diagnostics.ViewModels
         {
             if (_control is IAvaloniaObject ao)
             {
-                string CreateConstraintInfo(StyledProperty<double> minProperty, StyledProperty<double> maxProperty)
+                string? CreateConstraintInfo(StyledProperty<double> minProperty, StyledProperty<double> maxProperty)
                 {
                     bool hasMin = ao.IsSet(minProperty);
                     bool hasMax = ao.IsSet(maxProperty);
@@ -179,7 +179,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public void ControlPropertyChanged(object sender, AvaloniaPropertyChangedEventArgs e)
+        public void ControlPropertyChanged(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             try
             {

src/Avalonia.Diagnostics/Diagnostics/ViewModels/EventOwnerTreeNode.cs

@@ -27,7 +27,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
                     if (_updateChildren && value != null)
                     {
-                        foreach (var child in Children)
+                        foreach (var child in Children!)
                         {
                             try
                             {

src/Avalonia.Diagnostics/Diagnostics/ViewModels/EventTreeNode.cs

@@ -11,16 +11,13 @@ namespace Avalonia.Diagnostics.ViewModels
     {
         private readonly EventsPageViewModel _parentViewModel;
         private bool _isRegistered;
-        private FiredEvent _currentEvent;
+        private FiredEvent? _currentEvent;
 
         public EventTreeNode(EventOwnerTreeNode parent, RoutedEvent @event, EventsPageViewModel vm)
             : base(parent, @event.Name)
         {
-            Contract.Requires<ArgumentNullException>(@event != null);
-            Contract.Requires<ArgumentNullException>(vm != null);
-
-            Event = @event;
-            _parentViewModel = vm;
+            Event = @event ?? throw new ArgumentNullException(nameof(@event));
+            _parentViewModel = vm ?? throw new ArgumentNullException(nameof(vm));
         }
 
         public RoutedEvent Event { get; }
@@ -62,18 +59,18 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        private void HandleEvent(object sender, RoutedEventArgs e)
+        private void HandleEvent(object? sender, RoutedEventArgs e)
         {
             if (!_isRegistered || IsEnabled == false)
                 return;
             if (sender is IVisual v && BelongsToDevTool(v))
                 return;
 
-            var s = sender;
+            var s = sender!;
             var handled = e.Handled;
             var route = e.Route;
 
-            Action handler = delegate
+            void handler()
             {
                 if (_currentEvent == null || !_currentEvent.IsPartOfSameEventChain(e))
                 {
@@ -98,14 +95,16 @@ namespace Avalonia.Diagnostics.ViewModels
 
         private static bool BelongsToDevTool(IVisual v)
         {
-            while (v != null)
+            var current = v;
+
+            while (current != null)
             {
-                if (v is MainView || v is MainWindow)
+                if (current is MainView || current is MainWindow)
                 {
                     return true;
                 }
 
-                v = v.VisualParent;
+                current = current.VisualParent;
             }
 
             return false;

src/Avalonia.Diagnostics/Diagnostics/ViewModels/EventTreeNodeBase.cs

@@ -10,14 +10,14 @@ namespace Avalonia.Diagnostics.ViewModels
         private bool? _isEnabled = false;
         private bool _isVisible;
 
-        protected EventTreeNodeBase(EventTreeNodeBase parent, string text)
+        protected EventTreeNodeBase(EventTreeNodeBase? parent, string text)
         {
             Parent = parent;
             Text = text;
             IsVisible = true;
         }
 
-        public IAvaloniaReadOnlyList<EventTreeNodeBase> Children
+        public IAvaloniaReadOnlyList<EventTreeNodeBase>? Children
         {
             get;
             protected set;
@@ -41,7 +41,7 @@ namespace Avalonia.Diagnostics.ViewModels
             set => RaiseAndSetIfChanged(ref _isVisible, value);
         }
 
-        public EventTreeNodeBase Parent
+        public EventTreeNodeBase? Parent
         {
             get;
         }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/EventsPageViewModel.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
-using System.ComponentModel;
 using System.Linq;
 using Avalonia.Controls;
 using Avalonia.Diagnostics.Models;
@@ -23,8 +22,8 @@ namespace Avalonia.Diagnostics.ViewModels
         };
 
         private readonly MainViewModel _mainViewModel;
-        private FiredEvent _selectedEvent;
-        private EventTreeNodeBase _selectedNode;
+        private FiredEvent? _selectedEvent;
+        private EventTreeNodeBase? _selectedNode;
 
         public EventsPageViewModel(MainViewModel mainViewModel)
         {
@@ -48,13 +47,13 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public ObservableCollection<FiredEvent> RecordedEvents { get; } = new ObservableCollection<FiredEvent>();
 
-        public FiredEvent SelectedEvent
+        public FiredEvent? SelectedEvent
         {
             get => _selectedEvent;
             set => RaiseAndSetIfChanged(ref _selectedEvent, value);
         }
 
-        public EventTreeNodeBase SelectedNode
+        public EventTreeNodeBase? SelectedNode
         {
             get => _selectedNode;
             set => RaiseAndSetIfChanged(ref _selectedNode, value);
@@ -99,7 +98,7 @@ namespace Avalonia.Diagnostics.ViewModels
                 }
             }
 
-            static EventTreeNodeBase FindNode(EventTreeNodeBase node, RoutedEvent eventType)
+            static EventTreeNodeBase? FindNode(EventTreeNodeBase node, RoutedEvent eventType)
             {
                 if (node is EventTreeNode eventNode && eventNode.Event == eventType)
                 {

src/Avalonia.Diagnostics/Diagnostics/ViewModels/FilterViewModel.cs

@@ -11,10 +11,13 @@ namespace Avalonia.Diagnostics.ViewModels
         private readonly Dictionary<string, string> _errors = new Dictionary<string, string>();
         private string _filterString = string.Empty;
         private bool _useRegexFilter, _useCaseSensitiveFilter, _useWholeWordFilter;
-        private string _processedFilter;
-        private Regex _filterRegex;
+        private Regex? _filterRegex;
 
-        public event EventHandler RefreshFilter;
+        public event EventHandler? RefreshFilter;
+
+        public bool HasErrors => _errors.Count > 0;
+
+        public event EventHandler<DataErrorsChangedEventArgs>? ErrorsChanged;
 
         public bool Filter(string input)
         {
@@ -31,13 +34,11 @@ namespace Avalonia.Diagnostics.ViewModels
                 }
             }
 
-            _processedFilter = FilterString.Trim();
-
             try
             {
                 var options = RegexOptions.Compiled;
                 var pattern = UseRegexFilter
-                    ? _processedFilter : Regex.Escape(_processedFilter);
+                    ? FilterString.Trim() : Regex.Escape(FilterString.Trim());
                 if (!UseCaseSensitiveFilter)
                 {
                     options |= RegexOptions.IgnoreCase;
@@ -109,16 +110,13 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public IEnumerable GetErrors(string propertyName)
+        public IEnumerable GetErrors(string? propertyName)
         {
-            if (_errors.TryGetValue(propertyName, out var error))
+            if (propertyName != null
+                && _errors.TryGetValue(propertyName, out var error))
             {
                 yield return error;
             }
         }
-
-        public bool HasErrors => _errors.Count > 0;
-
-        public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/FiredEvent.cs

@@ -8,15 +8,12 @@ namespace Avalonia.Diagnostics.ViewModels
     internal class FiredEvent : ViewModelBase
     {
         private readonly RoutedEventArgs _eventArgs;
-        private EventChainLink _handledBy;
+        private EventChainLink? _handledBy;
 
         public FiredEvent(RoutedEventArgs eventArgs, EventChainLink originator)
         {
-            Contract.Requires<ArgumentNullException>(eventArgs != null);
-            Contract.Requires<ArgumentNullException>(originator != null);
-
-            _eventArgs = eventArgs;
-            Originator = originator;
+            _eventArgs = eventArgs ?? throw new ArgumentNullException(nameof(eventArgs));
+            Originator = originator ?? throw new ArgumentNullException(nameof(originator));
             AddToChain(originator);
         }
 
@@ -25,7 +22,7 @@ namespace Avalonia.Diagnostics.ViewModels
             return e == _eventArgs;
         }
 
-        public RoutedEvent Event => _eventArgs.RoutedEvent;
+        public RoutedEvent Event => _eventArgs.RoutedEvent!;
 
         public bool IsHandled => HandledBy?.Handled == true;
 
@@ -38,7 +35,7 @@ namespace Avalonia.Diagnostics.ViewModels
                 if (IsHandled)
                 {
                     return $"{Event.Name} on {Originator.HandlerName};" + Environment.NewLine +
-                           $"strategies: {Event.RoutingStrategies}; handled by: {HandledBy.HandlerName}";
+                           $"strategies: {Event.RoutingStrategies}; handled by: {HandledBy!.HandlerName}";
                 }
 
                 return $"{Event.Name} on {Originator.HandlerName}; strategies: {Event.RoutingStrategies}";
@@ -47,7 +44,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public EventChainLink Originator { get; }
 
-        public EventChainLink HandledBy
+        public EventChainLink? HandledBy
         {
             get => _handledBy;
             set

src/Avalonia.Diagnostics/Diagnostics/ViewModels/LogicalTreeNode.cs

@@ -7,22 +7,24 @@ namespace Avalonia.Diagnostics.ViewModels
 {
     internal class LogicalTreeNode : TreeNode
     {
-        public LogicalTreeNode(ILogical logical, TreeNode parent)
+        public LogicalTreeNode(ILogical logical, TreeNode? parent)
             : base((Control)logical, parent)
         {
             Children = new LogicalTreeNodeCollection(this, logical);
         }
 
+        public override TreeNodeCollection Children { get; }
+
         public static LogicalTreeNode[] Create(object control)
         {
             var logical = control as ILogical;
-            return logical != null ? new[] { new LogicalTreeNode(logical, null) } : null;
+            return logical != null ? new[] { new LogicalTreeNode(logical, null) } : Array.Empty<LogicalTreeNode>();
         }
 
         internal class LogicalTreeNodeCollection : TreeNodeCollection
         {
             private readonly ILogical _control;
-            private IDisposable _subscription;
+            private IDisposable? _subscription;
 
             public LogicalTreeNodeCollection(TreeNode owner, ILogical control)
                 : base(owner)

src/Avalonia.Diagnostics/Diagnostics/ViewModels/MainViewModel.cs

@@ -1,10 +1,10 @@
 using System;
 using System.ComponentModel;
+
 using Avalonia.Controls;
 using Avalonia.Diagnostics.Models;
 using Avalonia.Input;
 using Avalonia.Threading;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Diagnostics.ViewModels
 {
@@ -17,13 +17,16 @@ namespace Avalonia.Diagnostics.ViewModels
         private readonly IDisposable _pointerOverSubscription;
         private ViewModelBase _content;
         private int _selectedTab;
-        private string _focusedControl;
-        private string _pointerOverElement;
+        private string? _focusedControl;
+        private string? _pointerOverElement;
         private bool _shouldVisualizeMarginPadding = true;
         private bool _shouldVisualizeDirtyRects;
         private bool _showFpsOverlay;
 
+#nullable disable
+        // Remove "nullable disable" after MemberNotNull will work on our CI.
         public MainViewModel(TopLevel root)
+#nullable restore
         {
             _root = root;
             _logicalTree = new TreePageViewModel(this, LogicalTreeNode.Create(root));
@@ -84,6 +87,7 @@ namespace Avalonia.Diagnostics.ViewModels
         public ViewModelBase Content
         {
             get { return _content; }
+            // [MemberNotNull(nameof(_content))]
             private set
             {
                 if (_content is TreePageViewModel oldTree &&
@@ -114,39 +118,40 @@ namespace Avalonia.Diagnostics.ViewModels
         public int SelectedTab
         {
             get { return _selectedTab; }
+            // [MemberNotNull(nameof(_content))]
             set
             {
                 _selectedTab = value;
 
                 switch (value)
                 {
-                    case 0:
-                        Content = _logicalTree;
-                        break;
                     case 1:
                         Content = _visualTree;
                         break;
                     case 2:
                         Content = _events;
                         break;
+                    default:
+                        Content = _logicalTree;
+                        break;
                 }
 
                 RaisePropertyChanged();
             }
         }
 
-        public string FocusedControl
+        public string? FocusedControl
         {
             get { return _focusedControl; }
             private set { RaiseAndSetIfChanged(ref _focusedControl, value); }
         }
 
-        public string PointerOverElement
+        public string? PointerOverElement
         {
             get { return _pointerOverElement; }
             private set { RaiseAndSetIfChanged(ref _pointerOverElement, value); }
         }
-
+        
         private void UpdateConsoleContext(ConsoleContext context)
         {
             context.root = _root;
@@ -187,7 +192,7 @@ namespace Avalonia.Diagnostics.ViewModels
             FocusedControl = KeyboardDevice.Instance.FocusedElement?.GetType().Name;
         }
 
-        private void KeyboardPropertyChanged(object sender, PropertyChangedEventArgs e)
+        private void KeyboardPropertyChanged(object? sender, PropertyChangedEventArgs e)
         {
             if (e.PropertyName == nameof(KeyboardDevice.Instance.FocusedElement))
             {
@@ -208,5 +213,12 @@ namespace Avalonia.Diagnostics.ViewModels
                 tree.SelectControl(control);
             }
         }
+
+        public int? StartupScreenIndex { get; private set; } = default;
+
+        public void SetOptions(DevToolsOptions options)
+        {
+            StartupScreenIndex = options.StartupScreenIndex;
+        }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/PropertyViewModel.cs

@@ -16,9 +16,11 @@ namespace Avalonia.Diagnostics.ViewModels
         public abstract string Group { get; }
         public abstract string Type { get; }
         public abstract string Value { get; set; }
-        public abstract void Update();
+        public abstract string Priority { get; }
+        public abstract bool? IsAttached { get;  }
+        public abstract void Update();        
 
-        protected static string ConvertToString(object value)
+        protected static string ConvertToString(object? value)
         {
             if (value is null)
             {
@@ -31,13 +33,13 @@ namespace Avalonia.Diagnostics.ViewModels
             if (!converter.CanConvertTo(typeof(string)) || 
                 converter.GetType() == typeof(CollectionConverter))
             {
-                return value.ToString();
+                return value.ToString() ?? "(null)";
             }
 
             return converter.ConvertToString(value);
         }
 
-        private static object InvokeParse(string s, Type targetType)
+        private static object? InvokeParse(string s, Type targetType)
         {
             var method = targetType.GetMethod("Parse", PublicStatic, null, StringIFormatProviderParameters, null);
 
@@ -56,7 +58,7 @@ namespace Avalonia.Diagnostics.ViewModels
             throw new InvalidCastException("Unable to convert value.");
         }
 
-        protected static object ConvertFromString(string s, Type targetType)
+        protected static object? ConvertFromString(string s, Type targetType)
         {
             var converter = TypeDescriptor.GetConverter(targetType);
 

src/Avalonia.Diagnostics/Diagnostics/ViewModels/ResourceSetterViewModel.cs

@@ -8,7 +8,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public IBrush Tint { get; }
 
-        public ResourceSetterViewModel(AvaloniaProperty property, object resourceKey, object resourceValue, bool isDynamic) : base(property, resourceValue)
+        public ResourceSetterViewModel(AvaloniaProperty property, object resourceKey, object? resourceValue, bool isDynamic) : base(property, resourceValue)
         {
             Key = resourceKey;
             Tint = isDynamic ? Brushes.Orange : Brushes.Brown;
@@ -16,12 +16,14 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public void CopyResourceKey()
         {
-            if (Key is null)
+            var textToCopy = Key?.ToString();
+
+            if (textToCopy is null)
             {
                 return;
             }
 
-            CopyToClipboard(Key.ToString());
+            CopyToClipboard(textToCopy);
         }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/SetterViewModel.cs

@@ -11,7 +11,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public string Name { get; }
 
-        public object Value { get; }
+        public object? Value { get; }
 
         public bool IsActive
         {
@@ -25,7 +25,7 @@ namespace Avalonia.Diagnostics.ViewModels
             set => RaiseAndSetIfChanged(ref _isVisible, value);
         }
 
-        public SetterViewModel(AvaloniaProperty property, object value)
+        public SetterViewModel(AvaloniaProperty property, object? value)
         {
             Property = property;
             Name = property.Name;
@@ -36,12 +36,14 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public void CopyValue()
         {
-            if (Value is null)
+            var textToCopy = Value?.ToString();
+
+            if (textToCopy is null)
             {
                 return;
             }
 
-            CopyToClipboard(Value.ToString());
+            CopyToClipboard(textToCopy);
         }
 
         public void CopyPropertyName()

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreeNode.cs

@@ -9,17 +9,18 @@ using Avalonia.VisualTree;
 
 namespace Avalonia.Diagnostics.ViewModels
 {
-    internal class TreeNode : ViewModelBase, IDisposable
+    internal abstract class TreeNode : ViewModelBase, IDisposable
     {
-        private IDisposable _classesSubscription;
+        private IDisposable? _classesSubscription;
         private string _classes;
         private bool _isExpanded;
 
-        public TreeNode(IVisual visual, TreeNode parent)
+        public TreeNode(IVisual visual, TreeNode? parent)
         {
             Parent = parent;
             Type = visual.GetType().Name;
             Visual = visual;
+            _classes = string.Empty;
 
             if (visual is IControl control)
             {
@@ -51,10 +52,9 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public TreeNodeCollection Children
+        public abstract TreeNodeCollection Children
         {
             get;
-            protected set;
         }
 
         public string Classes
@@ -63,7 +63,7 @@ namespace Avalonia.Diagnostics.ViewModels
             private set { RaiseAndSetIfChanged(ref _classes, value); }
         }
 
-        public string ElementName
+        public string? ElementName
         {
             get;
         }
@@ -79,7 +79,7 @@ namespace Avalonia.Diagnostics.ViewModels
             set { RaiseAndSetIfChanged(ref _isExpanded, value); }
         }
 
-        public TreeNode Parent
+        public TreeNode? Parent
         {
             get;
         }
@@ -92,7 +92,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public void Dispose()
         {
-            _classesSubscription.Dispose();
+            _classesSubscription?.Dispose();
             Children.Dispose();
         }
 

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreeNodeCollection.cs

@@ -3,46 +3,33 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.ComponentModel;
+
 using Avalonia.Collections;
 
 namespace Avalonia.Diagnostics.ViewModels
 {
     internal abstract class TreeNodeCollection : IAvaloniaReadOnlyList<TreeNode>, IDisposable
     {
-        private AvaloniaList<TreeNode> _inner;
+        private AvaloniaList<TreeNode>? _inner;
 
         public TreeNodeCollection(TreeNode owner) => Owner = owner;
 
-        public TreeNode this[int index]
-        {
-            get
-            {
-                EnsureInitialized();
-                return _inner[index];
-            }
-        }
+        public TreeNode this[int index] => EnsureInitialized()[index];
 
-        public int Count
-        {
-            get
-            {
-                EnsureInitialized();
-                return _inner.Count;
-            }
-        }
+        public int Count => EnsureInitialized().Count;
 
         protected TreeNode Owner { get; }
 
-        public event NotifyCollectionChangedEventHandler CollectionChanged
+        public event NotifyCollectionChangedEventHandler? CollectionChanged
         {
-            add => _inner.CollectionChanged += value;
-            remove => _inner.CollectionChanged -= value;
+            add => EnsureInitialized().CollectionChanged += value;
+            remove => EnsureInitialized().CollectionChanged -= value;
         }
 
-        public event PropertyChangedEventHandler PropertyChanged
+        public event PropertyChangedEventHandler? PropertyChanged
         {
-            add => _inner.PropertyChanged += value;
-            remove => _inner.PropertyChanged -= value;
+            add => EnsureInitialized().PropertyChanged += value;
+            remove => EnsureInitialized().PropertyChanged -= value;
         }
 
         public virtual void Dispose()
@@ -58,21 +45,21 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public IEnumerator<TreeNode> GetEnumerator()
         {
-            EnsureInitialized();
-            return _inner.GetEnumerator();
+            return EnsureInitialized().GetEnumerator();
         }
 
         IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
         protected abstract void Initialize(AvaloniaList<TreeNode> nodes);
 
-        private void EnsureInitialized()
+        private AvaloniaList<TreeNode> EnsureInitialized()
         {
             if (_inner is null)
             {
                 _inner = new AvaloniaList<TreeNode>();
                 Initialize(_inner);
             }
+            return _inner;
         }
     }
 }

src/Avalonia.Diagnostics/Diagnostics/ViewModels/TreePageViewModel.cs

@@ -6,8 +6,8 @@ namespace Avalonia.Diagnostics.ViewModels
 {
     internal class TreePageViewModel : ViewModelBase, IDisposable
     {
-        private TreeNode _selectedNode;
-        private ControlDetailsViewModel _details;
+        private TreeNode? _selectedNode;
+        private ControlDetailsViewModel? _details;
 
         public TreePageViewModel(MainViewModel mainView, TreeNode[] nodes)
         {
@@ -29,7 +29,7 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public TreeNode[] Nodes { get; protected set; }
 
-        public TreeNode SelectedNode
+        public TreeNode? SelectedNode
         {
             get => _selectedNode;
             private set
@@ -44,7 +44,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        public ControlDetailsViewModel Details
+        public ControlDetailsViewModel? Details
         {
             get => _details;
             private set
@@ -68,7 +68,7 @@ namespace Avalonia.Diagnostics.ViewModels
             _details?.Dispose();
         }
 
-        public TreeNode FindNode(IControl control)
+        public TreeNode? FindNode(IControl control)
         {
             foreach (var node in Nodes)
             {
@@ -104,7 +104,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        private void ExpandNode(TreeNode node)
+        private void ExpandNode(TreeNode? node)
         {
             if (node != null)
             {
@@ -113,7 +113,7 @@ namespace Avalonia.Diagnostics.ViewModels
             }
         }
 
-        private TreeNode FindNode(TreeNode node, IControl control)
+        private TreeNode? FindNode(TreeNode node, IControl control)
         {
             if (node.Visual == control)
             {

src/Avalonia.Diagnostics/Diagnostics/ViewModels/ViewModelBase.cs

@@ -1,16 +1,16 @@
-using System;
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 
 namespace Avalonia.Diagnostics.ViewModels
 {
     internal class ViewModelBase : INotifyPropertyChanged
     {
-        private PropertyChangedEventHandler _propertyChanged;
+        private PropertyChangedEventHandler? _propertyChanged;
         private List<string> events = new List<string>();
 
-        public event PropertyChangedEventHandler PropertyChanged
+        public event PropertyChangedEventHandler? PropertyChanged
         {
             add { _propertyChanged += value; events.Add("added"); }
             remove { _propertyChanged -= value; events.Add("removed"); }
@@ -20,7 +20,7 @@ namespace Avalonia.Diagnostics.ViewModels
         {
         }
 
-        protected bool RaiseAndSetIfChanged<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
+        protected bool RaiseAndSetIfChanged<T>([NotNullIfNotNull("value")] ref T field, T value, [CallerMemberName] string propertyName = null!)
         {
             if (!EqualityComparer<T>.Default.Equals(field, value))
             {
@@ -32,7 +32,7 @@ namespace Avalonia.Diagnostics.ViewModels
             return false;
         }
 
-        protected void RaisePropertyChanged([CallerMemberName] string propertyName = null)
+        protected void RaisePropertyChanged([CallerMemberName] string propertyName = null!)
         {
             var e = new PropertyChangedEventArgs(propertyName);
             OnPropertyChanged(e);

src/Avalonia.Diagnostics/Diagnostics/ViewModels/VisualTreeNode.cs

@@ -7,7 +7,7 @@ namespace Avalonia.Diagnostics.ViewModels
 {
     internal class VisualTreeNode : TreeNode
     {
-        public VisualTreeNode(IVisual visual, TreeNode parent)
+        public VisualTreeNode(IVisual visual, TreeNode? parent)
             : base(visual, parent)
         {
             Children = new VisualTreeNodeCollection(this, visual);
@@ -20,16 +20,18 @@ namespace Avalonia.Diagnostics.ViewModels
 
         public bool IsInTemplate { get; private set; }
 
+        public override TreeNodeCollection Children { get; }
+
         public static VisualTreeNode[] Create(object control)
         {
             var visual = control as IVisual;
-            return visual != null ? new[] { new VisualTreeNode(visual, null) } : null;
+            return visual != null ? new[] { new VisualTreeNode(visual, null) } : Array.Empty<VisualTreeNode>();
         }
 
         internal class VisualTreeNodeCollection : TreeNodeCollection
         {
             private readonly IVisual _control;
-            private IDisposable _subscription;
+            private IDisposable? _subscription;
 
             public VisualTreeNodeCollection(TreeNode owner, IVisual control)
                 : base(owner)

src/Avalonia.Diagnostics/Diagnostics/Views/ConsoleView.xaml.cs

@@ -30,22 +30,26 @@ namespace Avalonia.Diagnostics.Views
             AvaloniaXamlLoader.Load(this);
         }
 
-        private void HistoryChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void HistoryChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
-            if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems[0] is IControl control)
+            if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems?[0] is IControl control)
             {
                 DispatcherTimer.RunOnce(control.BringIntoView, TimeSpan.Zero);
             }
         }
 
-        private void InputKeyDown(object sender, KeyEventArgs e)
+        private void InputKeyDown(object? sender, KeyEventArgs e)
         {
-            var vm = (ConsoleViewModel)DataContext;
+            var vm = (ConsoleViewModel?)DataContext;
+            if (vm is null)
+            {
+                return;
+            }
 
             switch (e.Key)
             {
                 case Key.Enter:
-                    vm.Execute();
+                    _ = vm.Execute();
                     e.Handled = true;
                     break;
                 case Key.Up: