almost working chrome api osx.

native/Avalonia.Native/inc/avalonia-native.h

@@ -204,6 +204,15 @@ enum AvnMenuItemToggleType
     Radio
 };
 
+enum AvnExtendClientAreaChromeHints
+{
+    AvnChromeHintsNoChrome,
+    AvnChromeHintsSystemTitleBar = 0x01,
+    AvnChromeHintsManagedChromeButtons = 0x02,
+    AvnChromeHintsPreferSystemChromeButtons = 0x04,
+    AvnChromeHintsOSXThickTitleBar = 0x08,
+};
+
 AVNCOM(IAvaloniaNativeFactory, 01) : IUnknown
 {
 public:
@@ -277,6 +286,8 @@ AVNCOM(IAvnWindow, 04) : virtual IAvnWindowBase
     virtual HRESULT SetWindowState(AvnWindowState state) = 0;
     virtual HRESULT GetWindowState(AvnWindowState*ret) = 0;
     virtual HRESULT SetExtendClientArea (bool enable) = 0;
+    virtual HRESULT SetExtendClientAreaHints (AvnExtendClientAreaChromeHints hints) = 0;
+    virtual HRESULT GetExtendTitleBarHeight (double*ret) = 0;
 };
 
 AVNCOM(IAvnWindowBaseEvents, 05) : IUnknown

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

@@ -475,6 +475,7 @@ private:
     NSRect _preZoomSize;
     bool _transitioningWindowState;
     bool _isClientAreaExtended;
+    AvnExtendClientAreaChromeHints _extendClientHints;
     
     FORWARD_IUNKNOWN()
     BEGIN_INTERFACE_MAP()
@@ -489,6 +490,7 @@ private:
     WindowImpl(IAvnWindowEvents* events, IAvnGlContext* gl) : WindowBaseImpl(events, gl)
     {
         _isClientAreaExtended = false;
+        _extendClientHints = AvnChromeHintsNoChrome;
         _fullScreenActive = false;
         _canResize = true;
         _decorations = SystemDecorationsFull;
@@ -509,6 +511,9 @@ private:
                 for (id button in titlebarView.subviews) {
                     if ([button isKindOfClass:[NSButton class]]) {
                         [button setHidden: (_decorations != SystemDecorationsFull)];
+                        [[button layer] setZPosition:5];
+                        [button setWantsLayer:true];
+                        
                     }
                 }
             }
@@ -771,25 +776,75 @@ private:
     virtual HRESULT SetExtendClientArea (bool enable) override
     {
         _isClientAreaExtended = enable;
-        [Window setTitleVisibility:NSWindowTitleHidden];
-        [Window setTitlebarAppearsTransparent:enable];
-        Window.movableByWindowBackground = true;
-        
-        NSRect x;
-        x.size.height = 50;
-        x.size.width = Window.frame.size.width;
-        [Window contentLayoutRect] = x;
         
+        if(enable)
+        {
+            Window.titleVisibility = NSWindowTitleHidden;
+            
+            if(_extendClientHints & AvnChromeHintsSystemTitleBar)
+            {
+                [Window setTitlebarAppearsTransparent:false];
+                View.layer.zPosition = 1;
+            }
+            else
+            {
+                [Window setTitlebarAppearsTransparent:true];
+                View.layer.zPosition = 0;
+            }
+            
+            if(_extendClientHints & AvnChromeHintsOSXThickTitleBar)
+            {
+                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;
+        }
         
-        auto customToolbar = [NSToolbar new];
+        UpdateStyle();
         
-        customToolbar.showsBaselineSeparator = true;
-        Window.toolbar = customToolbar;
+        return S_OK;
+    }
+    
+    virtual HRESULT SetExtendClientAreaHints (AvnExtendClientAreaChromeHints hints)
+    {
+        _extendClientHints = hints;
         
-        UpdateStyle();
+        SetExtendClientArea(_isClientAreaExtended);
         return S_OK;
     }
     
+    virtual HRESULT GetExtendTitleBarHeight (double*ret)
+    {
+        if(ret == nullptr)
+        {
+            return E_POINTER;
+        }
+        
+        for (id subview in Window.contentView.superview.subviews)
+        {
+            if ([subview isKindOfClass:NSClassFromString(@"NSTitlebarContainerView")])
+            {
+                NSView *titlebarView = [subview subviews][0];
+                
+                *ret = (double)titlebarView.frame.size.height;
+                
+                return S_OK;
+            }
+        }
+        
+        return E_FAIL;
+    }
+    
     void EnterFullScreenMode ()
     {
         _fullScreenActive = true;

src/Avalonia.Controls/Platform/ExtendClientAreaChromeHints.cs

@@ -10,6 +10,9 @@ namespace Avalonia.Platform
         SystemTitleBar = 0x01,
         ManagedChromeButtons = 0x02,
         PreferSystemChromeButtons = 0x04,
+
+        OSXThickTitleBar = 0x08,
+        
         AdaptiveChromeWithTitleBar = SystemTitleBar | PreferSystemChromeButtons,
         AdaptiveChromeWithoutTitleBar = PreferSystemChromeButtons,
     }

src/Avalonia.Native/WindowImpl.cs

@@ -102,7 +102,7 @@ namespace Avalonia.Native
 
         public Action<bool> ExtendClientAreaToDecorationsChanged { get; set; }
 
-        public Thickness ExtendedMargins { get; } = new Thickness(0, 20, 0, 0);
+        public Thickness ExtendedMargins { get; private set; }
 
         public Thickness OffScreenMargin { get; } = new Thickness();
 
@@ -137,12 +137,17 @@ namespace Avalonia.Native
         public void SetExtendClientAreaToDecorationsHint(bool extendIntoClientAreaHint)
         {
             _isExtended = extendIntoClientAreaHint;
+
             _native.SetExtendClientArea(extendIntoClientAreaHint);
+
+            ExtendedMargins = _isExtended ? new Thickness(0, _native.GetExtendTitleBarHeight(), 0, 0) : new Thickness();
+
             ExtendClientAreaToDecorationsChanged?.Invoke(true);
         }
 
         public void SetExtendClientAreaChromeHints(ExtendClientAreaChromeHints hints)
         {
+            _native.SetExtendClientAreaHints ((AvnExtendClientAreaChromeHints)hints);
         }
 
         public void SetExtendClientAreaTitleBarHeightHint(double titleBarHeight)