Fix auto-scaling on win32.

- Adds a "resize reason" to platform `Resized` events
- Which is used by the auto-sizing code to determine whether to reset `SizeToContent`
- Also other improvements to the reset logic for `SizeToContent`:
  - Don't reset it if the size hasn't changed
  - Don't reset it if `CanResize == false`
  - Only reset the dimensions whose size has changed
- Obsolete the `BeginAutoSizing`/`AutoSizing` feature in `WindowBase`

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

@@ -10,6 +10,8 @@ class WindowBaseImpl;
 -(void) setSwRenderedFrame: (AvnFramebuffer* _Nonnull) fb dispose: (IUnknown* _Nonnull) dispose;
 -(void) onClosed;
 -(AvnPixelSize) getPixelSize;
+-(AvnPlatformResizeReason) getResizeReason;
+-(void) setResizeReason:(AvnPlatformResizeReason)reason;
 @end
 
 @interface AutoFitContentView : NSView
@@ -50,4 +52,23 @@ struct IWindowStateChanged
     virtual AvnWindowState WindowState () = 0;
 };
 
+class ResizeScope
+{
+public:
+    ResizeScope(AvnView* _Nonnull view, AvnPlatformResizeReason reason)
+    {
+        _view = view;
+        _restore = [view getResizeReason];
+        [view setResizeReason:reason];
+    }
+    
+    ~ResizeScope()
+    {
+        [_view setResizeReason:_restore];
+    }
+private:
+    AvnView* _Nonnull _view;
+    AvnPlatformResizeReason _restore;
+};
+
 #endif /* window_h */

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

@@ -275,9 +275,10 @@ public:
         }
     }
     
-    virtual HRESULT Resize(double x, double y) override
+    virtual HRESULT Resize(double x, double y, AvnPlatformResizeReason reason) override
     {
         START_COM_CALL;
+        auto resizeBlock = ResizeScope(View, reason);
         
         @autoreleasepool
         {
@@ -306,7 +307,7 @@ public:
             
             if(!_shown)
             {
-                BaseEvents->Resized(AvnSize{x,y});
+                BaseEvents->Resized(AvnSize{x,y}, reason);
             }
             
             [StandardContainer setFrameSize:NSSize{x,y}];
@@ -1357,6 +1358,7 @@ NSArray* AllLoopModes = [NSArray arrayWithObjects: NSDefaultRunLoopMode, NSEvent
     bool _lastKeyHandled;
     AvnPixelSize _lastPixelSize;
     NSObject<IRenderTarget>* _renderTarget;
+    AvnPlatformResizeReason _resizeReason;
 }
 
 - (void)onClosed
@@ -1468,7 +1470,8 @@ NSArray* AllLoopModes = [NSArray arrayWithObjects: NSDefaultRunLoopMode, NSEvent
         _lastPixelSize.Height = (int)fsize.height;
         [self updateRenderTarget];
     
-        _parent->BaseEvents->Resized(AvnSize{newSize.width, newSize.height});
+        auto reason = [self inLiveResize] ? ResizeUser : _resizeReason;
+        _parent->BaseEvents->Resized(AvnSize{newSize.width, newSize.height}, reason);
     }
 }
 
@@ -1967,6 +1970,16 @@ NSArray* AllLoopModes = [NSArray arrayWithObjects: NSDefaultRunLoopMode, NSEvent
     
 }
 
+- (AvnPlatformResizeReason)getResizeReason
+{
+    return _resizeReason;
+}
+
+- (void)setResizeReason:(AvnPlatformResizeReason)reason
+{
+    _resizeReason = reason;
+}
+
 @end
 
 
@@ -2373,7 +2386,7 @@ protected:
         return NSWindowStyleMaskBorderless;
     }
     
-    virtual HRESULT Resize(double x, double y) override
+    virtual HRESULT Resize(double x, double y, AvnPlatformResizeReason reason) override
     {
         @autoreleasepool
         {

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

@@ -31,7 +31,7 @@ namespace Avalonia.Controls.Embedding.Offscreen
             set
             {
                 _clientSize = value;
-                Resized?.Invoke(value);
+                Resized?.Invoke(value, PlatformResizeReason.Unspecified);
             }
         }
 
@@ -49,7 +49,7 @@ namespace Avalonia.Controls.Embedding.Offscreen
         
         public Action<RawInputEventArgs> Input { get; set; }
         public Action<Rect> Paint { get; set; }
-        public Action<Size> Resized { get; set; }
+        public Action<Size, PlatformResizeReason> Resized { get; set; }
         public Action<double> ScalingChanged { get; set; }
 
         public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }

src/Avalonia.Controls/Platform/ITopLevelImpl.cs

@@ -3,11 +3,46 @@ using System.Collections.Generic;
 using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Input.Raw;
+using Avalonia.Layout;
 using Avalonia.Rendering;
 using JetBrains.Annotations;
 
 namespace Avalonia.Platform
 {
+    /// <summary>
+    /// Describes the reason for a <see cref="ITopLevelImpl.Resized"/> message.
+    /// </summary>
+    public enum PlatformResizeReason
+    {
+        /// <summary>
+        /// The resize reason is unknown or unspecified.
+        /// </summary>
+        Unspecified,
+
+        /// <summary>
+        /// The resize was due to the user resizing the window, for example by dragging the
+        /// window frame.
+        /// </summary>
+        User,
+
+        /// <summary>
+        /// The resize was initiated by the application, for example by setting one of the sizing-
+        /// related properties on <see cref="Window"/> such as <see cref="Layoutable.Width"/> or
+        /// <see cref="Layoutable.Height"/>.
+        /// </summary>
+        Application,
+
+        /// <summary>
+        /// The resize was initiated by the layout system.
+        /// </summary>
+        Layout,
+
+        /// <summary>
+        /// The resize was due to a change in DPI.
+        /// </summary>
+        DpiChange,
+    }
+
     /// <summary>
     /// Defines a platform-specific top-level window implementation.
     /// </summary>
@@ -57,7 +92,7 @@ namespace Avalonia.Platform
         /// <summary>
         /// Gets or sets a method called when the toplevel is resized.
         /// </summary>
-        Action<Size> Resized { get; set; }
+        Action<Size, PlatformResizeReason> Resized { get; set; }
 
         /// <summary>
         /// Gets or sets a method called when the toplevel's scaling changes.

src/Avalonia.Controls/Platform/IWindowImpl.cs

@@ -110,7 +110,9 @@ namespace Avalonia.Platform
         /// <summary>
         /// Sets the client size of the top level.
         /// </summary>
-        void Resize(Size clientSize);
+        /// <param name="clientSize">The new client size.</param>
+        /// <param name="reason">The reason for the resize.</param>
+        void Resize(Size clientSize, PlatformResizeReason reason = PlatformResizeReason.Application);
 
         /// <summary>
         /// Sets the client size of the top level.

src/Avalonia.Controls/Primitives/PopupRoot.cs

@@ -161,12 +161,9 @@ namespace Avalonia.Controls.Primitives
 
         protected override sealed Size ArrangeSetBounds(Size size)
         {
-            using (BeginAutoSizing())
-            {
-                _positionerParameters.Size = size;
-                UpdatePosition();
-                return ClientSize;
-            }
+            _positionerParameters.Size = size;
+            UpdatePosition();
+            return ClientSize;
         }
     }
 }

src/Avalonia.Controls/TopLevel.cs

@@ -380,7 +380,8 @@ namespace Avalonia.Controls
         /// Handles a resize notification from <see cref="ITopLevelImpl.Resized"/>.
         /// </summary>
         /// <param name="clientSize">The new client size.</param>
-        protected virtual void HandleResized(Size clientSize)
+        /// <param name="reason">The reason for the resize.</param>
+        protected virtual void HandleResized(Size clientSize, PlatformResizeReason reason)
         {
             ClientSize = clientSize;
             FrameSize = PlatformImpl.FrameSize;

src/Avalonia.Controls/Window.cs

@@ -244,7 +244,7 @@ namespace Avalonia.Controls
             impl.WindowStateChanged = HandleWindowStateChanged;
             _maxPlatformClientSize = PlatformImpl?.MaxAutoSizeHint ?? default(Size);
             impl.ExtendClientAreaToDecorationsChanged = ExtendClientAreaToDecorationsChanged;            
-            this.GetObservable(ClientSizeProperty).Skip(1).Subscribe(x => PlatformImpl?.Resize(x));
+            this.GetObservable(ClientSizeProperty).Skip(1).Subscribe(x => PlatformImpl?.Resize(x, PlatformResizeReason.Application));
 
             PlatformImpl?.ShowTaskbarIcon(ShowInTaskbar);
         }
@@ -258,6 +258,18 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets a value indicating how the window will size itself to fit its content.
         /// </summary>
+        /// <remarks>
+        /// If <see cref="SizeToContent"/> has a value other than <see cref="SizeToContent.Manual"/>,
+        /// <see cref="SizeToContent"/> is automatically set to <see cref="SizeToContent.Manual"/>
+        /// if a user resizes the window by using the resize grip or dragging the border.
+        /// 
+        /// NOTE: Because of a limitation of X11, <see cref="SizeToContent"/> will be reset on X11 to
+        /// <see cref="SizeToContent.Manual"/> on any resize - including the resize that happens when
+        /// the window is first shown. This is because X11 resize notifications are asynchronous and
+        /// there is no way to know whether a resize came from the user or the layout system. To avoid
+        /// this, consider setting <see cref="CanResize"/> to false, which will disable user resizing
+        /// of the window.
+        /// </remarks>
         public SizeToContent SizeToContent
         {
             get { return GetValue(SizeToContentProperty); }
@@ -583,28 +595,23 @@ namespace Avalonia.Controls
                 return;
             }
 
-            using (BeginAutoSizing())
-            {
-                Renderer?.Stop();
+            Renderer?.Stop();
 
-                if (Owner is Window owner)
-                {
-                    owner.RemoveChild(this);
-                }
+            if (Owner is Window owner)
+            {
+                owner.RemoveChild(this);
+            }
 
-                if (_children.Count > 0)
+            if (_children.Count > 0)
+            {
+                foreach (var child in _children.ToArray())
                 {
-                    foreach (var child in _children.ToArray())
-                    {
-                        child.child.Hide();
-                    }
+                    child.child.Hide();
                 }
-
-                Owner = null;
-
-                PlatformImpl?.Hide();
             }
 
+            Owner = null;
+            PlatformImpl?.Hide();
             IsVisible = false;
         }
 
@@ -675,29 +682,23 @@ namespace Avalonia.Controls
 
             if (initialSize != ClientSize)
             {
-                using (BeginAutoSizing())
-                {
-                    PlatformImpl?.Resize(initialSize);
-                }
+                PlatformImpl?.Resize(initialSize, PlatformResizeReason.Layout);
             }
 
             LayoutManager.ExecuteInitialLayoutPass();
 
-            using (BeginAutoSizing())
+            if (parent != null)
             {
-                if (parent != null)
-                {
-                    PlatformImpl?.SetParent(parent.PlatformImpl);
-                }
-                
-                Owner = parent;
-                parent?.AddChild(this, false);
-                
-                SetWindowStartupLocation(Owner?.PlatformImpl);
-                
-                PlatformImpl?.Show(ShowActivated);
-                Renderer?.Start();                
+                PlatformImpl?.SetParent(parent.PlatformImpl);
             }
+
+            Owner = parent;
+            parent?.AddChild(this, false);
+
+            SetWindowStartupLocation(Owner?.PlatformImpl);
+
+            PlatformImpl?.Show(ShowActivated);
+            Renderer?.Start();
             OnOpened(EventArgs.Empty);
         }
 
@@ -760,41 +761,34 @@ namespace Avalonia.Controls
 
             if (initialSize != ClientSize)
             {
-                using (BeginAutoSizing())
-                {
-                    PlatformImpl?.Resize(initialSize);
-                }
+                PlatformImpl?.Resize(initialSize, PlatformResizeReason.Layout);
             }
 
             LayoutManager.ExecuteInitialLayoutPass();
 
             var result = new TaskCompletionSource<TResult>();
 
-            using (BeginAutoSizing())
-            {
-                PlatformImpl?.SetParent(owner.PlatformImpl);
-                Owner = owner;
-                owner.AddChild(this, true);
-                
-                SetWindowStartupLocation(owner.PlatformImpl);
-                
-                PlatformImpl?.Show(ShowActivated);
+            PlatformImpl?.SetParent(owner.PlatformImpl);
+            Owner = owner;
+            owner.AddChild(this, true);
 
-                Renderer?.Start();
+            SetWindowStartupLocation(owner.PlatformImpl);
 
-                Observable.FromEventPattern<EventHandler, EventArgs>(
-                        x => Closed += x,
-                        x => Closed -= x)
-                    .Take(1)
-                    .Subscribe(_ =>
-                    {
-                        owner.Activate();
-                        result.SetResult((TResult)(_dialogResult ?? default(TResult)));
-                    });
+            PlatformImpl?.Show(ShowActivated);
 
-                OnOpened(EventArgs.Empty);
-            }
+            Renderer?.Start();
 
+            Observable.FromEventPattern<EventHandler, EventArgs>(
+                    x => Closed += x,
+                    x => Closed -= x)
+                .Take(1)
+                .Subscribe(_ =>
+                {
+                    owner.Activate();
+                    result.SetResult((TResult)(_dialogResult ?? default(TResult)));
+                });
+
+            OnOpened(EventArgs.Empty);
             return result.Task;
         }
 
@@ -937,11 +931,8 @@ namespace Avalonia.Controls
 
         protected sealed override Size ArrangeSetBounds(Size size)
         {
-            using (BeginAutoSizing())
-            {
-                PlatformImpl?.Resize(size);
-                return ClientSize;
-            }
+            PlatformImpl?.Resize(size, PlatformResizeReason.Layout);
+            return ClientSize;
         }
 
         protected sealed override void HandleClosed()
@@ -959,17 +950,32 @@ namespace Avalonia.Controls
         }
 
         /// <inheritdoc/>
-        protected sealed override void HandleResized(Size clientSize)
+        protected sealed override void HandleResized(Size clientSize, PlatformResizeReason reason)
         {
-            if (!AutoSizing)
+            if (ClientSize == clientSize)
+                return;
+
+            var sizeToContent = SizeToContent;
+
+            // If auto-sizing is enabled, and the resize came from a user resize (or the reason was
+            // unspecified) then turn off auto-resizing for any window dimension that is not equal
+            // to the requested size.
+            if (sizeToContent != SizeToContent.Manual &&
+                CanResize &&
+                reason == PlatformResizeReason.Unspecified ||  
+                reason == PlatformResizeReason.User)
             {
-                SizeToContent = SizeToContent.Manual;
+                if (clientSize.Width != ClientSize.Width)
+                    sizeToContent &= ~SizeToContent.Width;
+                if (clientSize.Height != ClientSize.Height)
+                    sizeToContent &= ~SizeToContent.Height;
+                SizeToContent = sizeToContent;
             }
 
             Width = clientSize.Width;
             Height = clientSize.Height;
 
-            base.HandleResized(clientSize);
+            base.HandleResized(clientSize, reason);
         }
 
         /// <summary>

src/Avalonia.Controls/WindowBase.cs

@@ -39,7 +39,6 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<bool> TopmostProperty =
             AvaloniaProperty.Register<WindowBase, bool>(nameof(Topmost));
 
-        private int _autoSizing;
         private bool _hasExecutedInitialLayoutPass;
         private bool _isActive;
         private bool _ignoreVisibilityChange;
@@ -95,10 +94,8 @@ namespace Avalonia.Controls
         
         public Screens Screens { get; private set; }
 
-        /// <summary>
-        /// Whether an auto-size operation is in progress.
-        /// </summary>
-        protected bool AutoSizing => _autoSizing > 0;
+        [Obsolete("No longer used. Always returns false.")]
+        protected bool AutoSizing => false;
 
         /// <summary>
         /// Gets or sets the owner of the window.
@@ -172,20 +169,9 @@ namespace Avalonia.Controls
             }
         }
 
-        /// <summary>
-        /// Begins an auto-resize operation.
-        /// </summary>
-        /// <returns>A disposable used to finish the operation.</returns>
-        /// <remarks>
-        /// When an auto-resize operation is in progress any resize events received will not be
-        /// cause the new size to be written to the <see cref="Layoutable.Width"/> and
-        /// <see cref="Layoutable.Height"/> properties.
-        /// </remarks>
-        protected IDisposable BeginAutoSizing()
-        {
-            ++_autoSizing;
-            return Disposable.Create(() => --_autoSizing);
-        }
+
+        [Obsolete("No longer used. Has no effect.")]
+        protected IDisposable BeginAutoSizing() => Disposable.Empty;
 
         /// <summary>
         /// Ensures that the window is initialized.
@@ -219,7 +205,8 @@ namespace Avalonia.Controls
         /// Handles a resize notification from <see cref="ITopLevelImpl.Resized"/>.
         /// </summary>
         /// <param name="clientSize">The new client size.</param>
-        protected override void HandleResized(Size clientSize)
+        /// <param name="reason">The reason for the resize.</param>
+        protected override void HandleResized(Size clientSize, PlatformResizeReason reason)
         {
             ClientSize = clientSize;
             FrameSize = PlatformImpl.FrameSize;

src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs

@@ -58,7 +58,7 @@ namespace Avalonia.DesignerSupport.Remote
             base.OnMessage(transport, obj);
         }
         
-        public void Resize(Size clientSize)
+        public void Resize(Size clientSize, PlatformResizeReason reason)
         {
             _transport.Send(new RequestViewportResizeMessage
             {

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -27,7 +27,7 @@ namespace Avalonia.DesignerSupport.Remote
         public IEnumerable<object> Surfaces { get; }
         public Action<RawInputEventArgs> Input { get; set; }
         public Action<Rect> Paint { get; set; }
-        public Action<Size> Resized { get; set; }
+        public Action<Size, PlatformResizeReason> Resized { get; set; }
         public Action<double> ScalingChanged { get; set; }
         public Func<bool> Closing { get; set; }
         public Action Closed { get; set; }
@@ -54,7 +54,7 @@ namespace Avalonia.DesignerSupport.Remote
                 PopupPositioner = new ManagedPopupPositioner(new ManagedPopupPositionerPopupImplHelper(parent,
                     (_, size, __) =>
                     {
-                        Resize(size);
+                        Resize(size, PlatformResizeReason.Unspecified);
                     }));
         }
 
@@ -98,7 +98,7 @@ namespace Avalonia.DesignerSupport.Remote
         {
         }
 
-        public void Resize(Size clientSize)
+        public void Resize(Size clientSize, PlatformResizeReason reason)
         {
         }
 

src/Avalonia.Headless/HeadlessWindowImpl.cs

@@ -47,7 +47,7 @@ namespace Avalonia.Headless
         public IEnumerable<object> Surfaces { get; }
         public Action<RawInputEventArgs> Input { get; set; }
         public Action<Rect> Paint { get; set; }
-        public Action<Size> Resized { get; set; }
+        public Action<Size, PlatformResizeReason> Resized { get; set; }
         public Action<double> ScalingChanged { get; set; }
 
         public IRenderer CreateRenderer(IRenderRoot root)
@@ -108,7 +108,7 @@ namespace Avalonia.Headless
         public Action Activated { get; set; }
         public IPlatformHandle Handle { get; } = new PlatformHandle(IntPtr.Zero, "STUB");
         public Size MaxClientSize { get; } = new Size(1920, 1280);
-        public void Resize(Size clientSize)
+        public void Resize(Size clientSize, PlatformResizeReason reason)
         {
             // Emulate X11 behavior here
             if (IsPopup)
@@ -126,7 +126,7 @@ namespace Avalonia.Headless
             if (ClientSize != clientSize)
             {
                 ClientSize = clientSize;
-                Resized?.Invoke(clientSize);
+                Resized?.Invoke(clientSize, PlatformResizeReason.Unspecified);
             }
         }
 

src/Avalonia.Native/PopupImpl.cs

@@ -32,7 +32,7 @@ namespace Avalonia.Native
         private void MoveResize(PixelPoint position, Size size, double scaling)
         {
             Position = position;
-            Resize(size);
+            Resize(size, PlatformResizeReason.Layout);
             //TODO: We ignore the scaling override for now
         }
 

src/Avalonia.Native/WindowImplBase.cs

@@ -87,7 +87,7 @@ namespace Avalonia.Native
             var monitor = Screen.AllScreens.OrderBy(x => x.PixelDensity)
                     .FirstOrDefault(m => m.Bounds.Contains(Position));
 
-            Resize(new Size(monitor.WorkingArea.Width * 0.75d, monitor.WorkingArea.Height * 0.7d));
+            Resize(new Size(monitor.WorkingArea.Width * 0.75d, monitor.WorkingArea.Height * 0.7d), PlatformResizeReason.Layout);
         }
 
         public Size ClientSize 
@@ -146,7 +146,7 @@ namespace Avalonia.Native
         public Action LostFocus { get; set; }
         
         public Action<Rect> Paint { get; set; }
-        public Action<Size> Resized { get; set; }
+        public Action<Size, PlatformResizeReason> Resized { get; set; }
         public Action Closed { get; set; }
         public IMouseDevice MouseDevice => _mouse;
         public abstract IPopupImpl CreatePopup();
@@ -186,13 +186,13 @@ namespace Avalonia.Native
                 _parent.Paint?.Invoke(new Rect(0, 0, s.Width, s.Height));
             }
 
-            void IAvnWindowBaseEvents.Resized(AvnSize* size)
+            void IAvnWindowBaseEvents.Resized(AvnSize* size, AvnPlatformResizeReason reason)
             {
                 if (_parent?._native != null)
                 {
                     var s = new Size(size->Width, size->Height);
                     _parent._savedLogicalSize = s;
-                    _parent.Resized?.Invoke(s);
+                    _parent.Resized?.Invoke(s, (PlatformResizeReason)reason);
                 }
             }
 
@@ -320,9 +320,9 @@ namespace Avalonia.Native
             }
         }
 
-        public void Resize(Size clientSize)
+        public void Resize(Size clientSize, PlatformResizeReason reason)
         {
-            _native.Resize(clientSize.Width, clientSize.Height);
+            _native.Resize(clientSize.Width, clientSize.Height, (AvnPlatformResizeReason)reason);
         }
 
         public IRenderer CreateRenderer(IRenderRoot root)

src/Avalonia.Native/avn.idl

@@ -400,6 +400,15 @@ enum AvnExtendClientAreaChromeHints
     AvnDefaultChrome = AvnPreferSystemChrome,
 }
 
+enum AvnPlatformResizeReason
+{
+    ResizeUnspecified,
+    ResizeUser,
+    ResizeApplication,
+    ResizeLayout,
+    ResizeDpiChange,
+}
+
 [uuid(809c652e-7396-11d2-9771-00a0c9b4d50c)]
 interface IAvaloniaNativeFactory : IUnknown
 {
@@ -438,7 +447,7 @@ interface IAvnWindowBase : IUnknown
      HRESULT GetFrameSize(AvnSize*ret);
      HRESULT GetScaling(double*ret);
      HRESULT SetMinMaxSize(AvnSize minSize, AvnSize maxSize);
-     HRESULT Resize(double width, double height);
+     HRESULT Resize(double width, double height, AvnPlatformResizeReason reason);
      HRESULT Invalidate(AvnRect rect);
      HRESULT BeginMoveDrag();
      HRESULT BeginResizeDrag(AvnWindowEdge edge);
@@ -492,7 +501,7 @@ interface IAvnWindowBaseEvents : IUnknown
      void Closed();
      void Activated();
      void Deactivated();
-     void Resized([const] AvnSize& size);
+     void Resized([const] AvnSize& size, AvnPlatformResizeReason reason);
      void PositionChanged(AvnPoint position);
      void RawMouseEvent(AvnRawMouseEventType type,
                                 uint timeStamp,

src/Avalonia.X11/X11Window.cs

@@ -336,7 +336,7 @@ namespace Avalonia.X11
         public IEnumerable<object> Surfaces { get; }
         public Action<RawInputEventArgs> Input { get; set; }
         public Action<Rect> Paint { get; set; }
-        public Action<Size> Resized { get; set; }
+        public Action<Size, PlatformResizeReason> Resized { get; set; }
         //TODO
         public Action<double> ScalingChanged { get; set; }
         public Action Deactivated { get; set; }
@@ -510,7 +510,7 @@ namespace Avalonia.X11
                         UpdateImePosition();
 
                         if (changedSize && !updatedSizeViaScaling && !_popup)
-                            Resized?.Invoke(ClientSize);
+                            Resized?.Invoke(ClientSize, PlatformResizeReason.Unspecified);
 
                         Dispatcher.UIThread.RunJobs(DispatcherPriority.Layout);
                     }, DispatcherPriority.Layout);
@@ -565,7 +565,7 @@ namespace Avalonia.X11
                     UpdateImePosition();
                     SetMinMaxSize(_scaledMinMaxSize.minSize, _scaledMinMaxSize.maxSize);
                     if(!skipResize)
-                        Resize(oldScaledSize, true);
+                        Resize(oldScaledSize, true, PlatformResizeReason.DpiChange);
                     return true;
                 }
 
@@ -616,7 +616,7 @@ namespace Avalonia.X11
             {
                 // Occurs once the window has been mapped, which is the earliest the extents
                 // can be retrieved, so invoke event to force update of TopLevel.FrameSize.
-                Resized.Invoke(ClientSize);
+                Resized.Invoke(ClientSize, PlatformResizeReason.Unspecified);
             }
 
             if (atom == _x11.Atoms._NET_WM_STATE)
@@ -862,19 +862,19 @@ namespace Avalonia.X11
         }
 
 
-        public void Resize(Size clientSize) => Resize(clientSize, false);
+        public void Resize(Size clientSize, PlatformResizeReason reason) => Resize(clientSize, false, reason);
         public void Move(PixelPoint point) => Position = point;
         private void MoveResize(PixelPoint position, Size size, double scaling)
         {
             Move(position);
             _scalingOverride = scaling;
             UpdateScaling(true);
-            Resize(size, true);
+            Resize(size, true, PlatformResizeReason.Layout);
         }
 
         PixelSize ToPixelSize(Size size) => new PixelSize((int)(size.Width * RenderScaling), (int)(size.Height * RenderScaling));
         
-        void Resize(Size clientSize, bool force)
+        void Resize(Size clientSize, bool force, PlatformResizeReason reason)
         {
             if (!force && clientSize == ClientSize)
                 return;
@@ -891,7 +891,7 @@ namespace Avalonia.X11
             if (force || !_wasMappedAtLeastOnce || (_popup && needImmediatePopupResize))
             {
                 _realSize = pixelSize;
-                Resized?.Invoke(ClientSize);
+                Resized?.Invoke(ClientSize, reason);
             }
         }
         

src/Linux/Avalonia.LinuxFramebuffer/FramebufferToplevelImpl.cs

@@ -69,7 +69,7 @@ namespace Avalonia.LinuxFramebuffer
         public IEnumerable<object> Surfaces { get; }
         public Action<RawInputEventArgs> Input { get; set; }
         public Action<Rect> Paint { get; set; }
-        public Action<Size> Resized { get; set; }
+        public Action<Size, PlatformResizeReason> Resized { get; set; }
         public Action<double> ScalingChanged { get; set; }
 
         public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }

src/Windows/Avalonia.Win32.Interop/Wpf/WpfTopLevelImpl.cs

@@ -44,7 +44,7 @@ namespace Avalonia.Win32.Interop.Wpf
                 ((FrameworkElement)PlatformImpl)?.InvalidateMeasure();
             }
 
-            protected override void HandleResized(Size clientSize)
+            protected override void HandleResized(Size clientSize, PlatformResizeReason reason)
             {
                 ClientSize = clientSize;
                 LayoutManager.ExecuteLayoutPass();
@@ -114,7 +114,7 @@ namespace Avalonia.Win32.Interop.Wpf
             if (_finalSize == _previousSize)
                 return finalSize;
             _previousSize = _finalSize;
-            _ttl.Resized?.Invoke(finalSize.ToAvaloniaSize());
+            _ttl.Resized?.Invoke(finalSize.ToAvaloniaSize(), PlatformResizeReason.Unspecified);
             return base.ArrangeOverride(finalSize);
         }
 
@@ -236,7 +236,7 @@ namespace Avalonia.Win32.Interop.Wpf
 
         Action<RawInputEventArgs> ITopLevelImpl.Input { get; set; } //TODO
         Action<Rect> ITopLevelImpl.Paint { get; set; }
-        Action<Size> ITopLevelImpl.Resized { get; set; }
+        Action<Size, PlatformResizeReason> ITopLevelImpl.Resized { get; set; }
         Action<double> ITopLevelImpl.ScalingChanged { get; set; }
 
         Action<WindowTransparencyLevel> ITopLevelImpl.TransparencyLevelChanged { get; set; }

src/Windows/Avalonia.Win32/PopupImpl.cs

@@ -135,7 +135,7 @@ namespace Avalonia.Win32
         private void MoveResize(PixelPoint position, Size size, double scaling)
         {
             Move(position);
-            Resize(size);
+            Resize(size, PlatformResizeReason.Layout);
             //TODO: We ignore the scaling override for now
         }
 

src/Windows/Avalonia.Win32/WindowImpl.AppWndProc.cs

@@ -2,9 +2,10 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.InteropServices;
 using Avalonia.Controls;
-using Avalonia.Controls.Platform;
+using Avalonia.Controls.Remote;
 using Avalonia.Input;
 using Avalonia.Input.Raw;
+using Avalonia.Platform;
 using Avalonia.Win32.Input;
 using static Avalonia.Win32.Interop.UnmanagedMethods;
 
@@ -18,7 +19,6 @@ namespace Avalonia.Win32
         {
             const double wheelDelta = 120.0;
             uint timestamp = unchecked((uint)GetMessageTime());
-
             RawInputEventArgs e = null;
             var shouldTakeFocus = false;
 
@@ -94,14 +94,19 @@ namespace Avalonia.Win32
                         var newDisplayRect = Marshal.PtrToStructure<RECT>(lParam);
                         _scaling = dpi / 96.0;
                         ScalingChanged?.Invoke(_scaling);
-                        SetWindowPos(hWnd,
-                            IntPtr.Zero,
-                            newDisplayRect.left,
-                            newDisplayRect.top,
-                            newDisplayRect.right - newDisplayRect.left,
-                            newDisplayRect.bottom - newDisplayRect.top,
-                            SetWindowPosFlags.SWP_NOZORDER |
-                            SetWindowPosFlags.SWP_NOACTIVATE);
+                        
+                        using (SetResizeReason(PlatformResizeReason.DpiChange))
+                        { 
+                            SetWindowPos(hWnd,
+                                IntPtr.Zero,
+                                newDisplayRect.left,
+                                newDisplayRect.top,
+                                newDisplayRect.right - newDisplayRect.left,
+                                newDisplayRect.bottom - newDisplayRect.top,
+                                SetWindowPosFlags.SWP_NOZORDER |
+                                SetWindowPosFlags.SWP_NOACTIVATE);
+                        }
+
                         return IntPtr.Zero;
                     }
 
@@ -364,6 +369,11 @@ namespace Avalonia.Win32
                     return IntPtr.Zero;
                 }
 
+
+                case WindowsMessage.WM_ENTERSIZEMOVE:
+                    _resizeReason = PlatformResizeReason.User;
+                    break;
+
                 case WindowsMessage.WM_SIZE:
                     {
                         using(NonPumpingSyncContext.Use())
@@ -379,7 +389,7 @@ namespace Avalonia.Win32
                              size == SizeCommand.Maximized))
                         {
                             var clientSize = new Size(ToInt32(lParam) & 0xffff, ToInt32(lParam) >> 16);
-                            Resized(clientSize / RenderScaling);
+                            Resized(clientSize / RenderScaling, _resizeReason);
                         }
 
                         var windowState = size == SizeCommand.Maximized ?
@@ -403,6 +413,10 @@ namespace Avalonia.Win32
                         return IntPtr.Zero;
                     }
 
+                case WindowsMessage.WM_EXITSIZEMOVE:
+                    _resizeReason = PlatformResizeReason.Unspecified;
+                    break;
+
                 case WindowsMessage.WM_MOVE:
                     {
                         PositionChanged?.Invoke(new PixelPoint((short)(ToInt32(lParam) & 0xffff),

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -53,6 +53,7 @@ namespace Avalonia.Win32
         private double _extendTitleBarHint = -1;
         private bool _isUsingComposition;
         private IBlurHost _blurHost;
+        private PlatformResizeReason _resizeReason;
 
 #if USE_MANAGED_DRAG
         private readonly ManagedWindowResizeDragHelper _managedDrag;
@@ -160,7 +161,7 @@ namespace Avalonia.Win32
 
         public Action<Rect> Paint { get; set; }
 
-        public Action<Size> Resized { get; set; }
+        public Action<Size, PlatformResizeReason> Resized { get; set; }
 
         public Action<double> ScalingChanged { get; set; }
 
@@ -482,7 +483,7 @@ namespace Avalonia.Win32
                 : new ImmediateRenderer(root);
         }
 
-        public void Resize(Size value)
+        public void Resize(Size value, PlatformResizeReason reason)
         {
             int requestedClientWidth = (int)(value.Width * RenderScaling);
             int requestedClientHeight = (int)(value.Height * RenderScaling);
@@ -494,6 +495,7 @@ namespace Avalonia.Win32
             {
                 GetWindowRect(_hwnd, out var windowRect);
 
+                using var scope = SetResizeReason(reason);
                 SetWindowPos(
                     _hwnd,
                     IntPtr.Zero,
@@ -930,7 +932,7 @@ namespace Avalonia.Win32
                 _offScreenMargin = new Thickness();
                 _extendedMargins = new Thickness();
                 
-                Resize(new Size(rcWindow.Width/ RenderScaling, rcWindow.Height / RenderScaling));
+                Resize(new Size(rcWindow.Width/ RenderScaling, rcWindow.Height / RenderScaling), PlatformResizeReason.Layout);
             }
 
             if(!_isClientAreaExtended || (_extendChromeHints.HasAllFlags(ExtendClientAreaChromeHints.SystemChrome) &&
@@ -1311,6 +1313,13 @@ namespace Avalonia.Win32
         /// <inheritdoc/>
         public AcrylicPlatformCompensationLevels AcrylicCompensationLevels { get; } = new AcrylicPlatformCompensationLevels(1, 0.8, 0);
 
+        private ResizeReasonScope SetResizeReason(PlatformResizeReason reason)
+        {
+            var old = _resizeReason;
+            _resizeReason = reason;
+            return new ResizeReasonScope(this, old);
+        }
+
         private struct SavedWindowInfo
         {
             public WindowStyles Style { get; set; }
@@ -1325,5 +1334,19 @@ namespace Avalonia.Win32
             public SystemDecorations Decorations;
             public bool IsFullScreen;
         }
+
+        private struct ResizeReasonScope : IDisposable
+        {
+            private readonly WindowImpl _owner;
+            private readonly PlatformResizeReason _restore;
+            
+            public ResizeReasonScope(WindowImpl owner, PlatformResizeReason restore)
+            {
+                _owner = owner;
+                _restore = restore;
+            }
+
+            public void Dispose() => _owner._resizeReason = _restore;
+        }
     }
 }

src/iOS/Avalonia.iOS/AvaloniaView.cs

@@ -96,7 +96,7 @@ namespace Avalonia.iOS
             public IEnumerable<object> Surfaces { get; set; }
             public Action<RawInputEventArgs> Input { get; set; }
             public Action<Rect> Paint { get; set; }
-            public Action<Size> Resized { get; set; }
+            public Action<Size, PlatformResizeReason> Resized { get; set; }
             public Action<double> ScalingChanged { get; set; }
             public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
             public Action Closed { get; set; }

tests/Avalonia.Controls.UnitTests/TopLevelTests.cs

@@ -139,7 +139,7 @@ namespace Avalonia.Controls.UnitTests
 
                 // The user has resized the window, so we can no longer auto-size.
                 var target = new TestTopLevel(impl.Object);
-                impl.Object.Resized(new Size(100, 200));
+                impl.Object.Resized(new Size(100, 200), PlatformResizeReason.Unspecified);
 
                 Assert.Equal(100, target.Width);
                 Assert.Equal(200, target.Height);

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -663,10 +663,11 @@ namespace Avalonia.Controls.UnitTests
                     var clientSize = new Size(200, 200);
                     var maxClientSize = new Size(480, 480);
 
-                    windowImpl.Setup(x => x.Resize(It.IsAny<Size>())).Callback<Size>(size =>
+                    windowImpl.Setup(x => x.Resize(It.IsAny<Size>(), It.IsAny<PlatformResizeReason>()))
+                        .Callback<Size, PlatformResizeReason>((size, reason) =>
                     {
                         clientSize = size.Constrain(maxClientSize);
-                        windowImpl.Object.Resized?.Invoke(clientSize);
+                        windowImpl.Object.Resized?.Invoke(clientSize, reason);
                     });
 
                     windowImpl.Setup(x => x.ClientSize).Returns(() => clientSize);
@@ -739,6 +740,36 @@ namespace Avalonia.Controls.UnitTests
                 }
             }
 
+            [Fact]
+            public void SizeToContent_Should_Not_Be_Lost_On_Scaling_Change()
+            {
+                using (UnitTestApplication.Start(TestServices.StyledWindow))
+                {
+                    var child = new Canvas
+                    {
+                        Width = 209,
+                        Height = 117,
+                    };
+
+                    var target = new Window()
+                    {
+                        SizeToContent = SizeToContent.WidthAndHeight,
+                        Content = child
+                    };
+
+                    Show(target);
+
+                    // Size before and after DPI change is a real-world example, with size after DPI
+                    // change coming from Win32 WM_DPICHANGED.
+                    target.PlatformImpl.ScalingChanged(1.5);
+                    target.PlatformImpl.Resized(
+                        new Size(210.66666666666666, 118.66666666666667),
+                        PlatformResizeReason.DpiChange);
+
+                    Assert.Equal(SizeToContent.WidthAndHeight, target.SizeToContent);
+                }
+            }
+
             [Fact]
             public void Width_Height_Should_Be_Updated_When_SizeToContent_Is_WidthAndHeight()
             {
@@ -791,8 +822,91 @@ namespace Avalonia.Controls.UnitTests
                     target.LayoutManager.ExecuteLayoutPass();
 
                     var windowImpl = Mock.Get(target.PlatformImpl);
-                    windowImpl.Verify(x => x.Resize(new Size(410, 800)));
+                    windowImpl.Verify(x => x.Resize(new Size(410, 800), PlatformResizeReason.Application));
+                    Assert.Equal(410, target.Width);
+                }
+            }
+
+
+            [Fact]
+            public void User_Resize_Of_Window_Width_Should_Reset_SizeToContent()
+            {
+                using (UnitTestApplication.Start(TestServices.StyledWindow))
+                {
+                    var target = new Window()
+                    {
+                        SizeToContent = SizeToContent.WidthAndHeight,
+                        Content = new Canvas
+                        {
+                            Width = 400,
+                            Height = 800,
+                        },
+                    };
+
+                    Show(target);
+                    Assert.Equal(400, target.Width);
+                    Assert.Equal(800, target.Height);
+
+                    target.PlatformImpl.Resized(new Size(410, 800), PlatformResizeReason.User);
+
                     Assert.Equal(410, target.Width);
+                    Assert.Equal(800, target.Height);
+                    Assert.Equal(SizeToContent.Height, target.SizeToContent);
+                }
+            }
+
+            [Fact]
+            public void User_Resize_Of_Window_Height_Should_Reset_SizeToContent()
+            {
+                using (UnitTestApplication.Start(TestServices.StyledWindow))
+                {
+                    var target = new Window()
+                    {
+                        SizeToContent = SizeToContent.WidthAndHeight,
+                        Content = new Canvas
+                        {
+                            Width = 400,
+                            Height = 800,
+                        },
+                    };
+
+                    Show(target);
+                    Assert.Equal(400, target.Width);
+                    Assert.Equal(800, target.Height);
+
+                    target.PlatformImpl.Resized(new Size(400, 810), PlatformResizeReason.User);
+
+                    Assert.Equal(400, target.Width);
+                    Assert.Equal(810, target.Height);
+                    Assert.Equal(SizeToContent.Width, target.SizeToContent);
+                }
+            }
+
+            [Fact]
+            public void Window_Resize_Should_Not_Reset_SizeToContent_If_CanResize_False()
+            {
+                using (UnitTestApplication.Start(TestServices.StyledWindow))
+                {
+                    var target = new Window()
+                    {
+                        SizeToContent = SizeToContent.WidthAndHeight,
+                        CanResize = false,
+                        Content = new Canvas
+                        {
+                            Width = 400,
+                            Height = 800,
+                        },
+                    };
+
+                    Show(target);
+                    Assert.Equal(400, target.Width);
+                    Assert.Equal(800, target.Height);
+
+                    target.PlatformImpl.Resized(new Size(410, 810), PlatformResizeReason.Unspecified);
+
+                    Assert.Equal(400, target.Width);
+                    Assert.Equal(800, target.Height);
+                    Assert.Equal(SizeToContent.WidthAndHeight, target.SizeToContent);
                 }
             }
 

tests/Avalonia.UnitTests/MockWindowingPlatform.cs

@@ -52,10 +52,11 @@ namespace Avalonia.UnitTests
                 windowImpl.Object.PositionChanged?.Invoke(x);
             });
 
-            windowImpl.Setup(x => x.Resize(It.IsAny<Size>())).Callback<Size>(x =>
+            windowImpl.Setup(x => x.Resize(It.IsAny<Size>(), It.IsAny<PlatformResizeReason>()))
+                .Callback<Size, PlatformResizeReason>((x, y) =>
             {
                 clientSize = x.Constrain(s_screenSize);
-                windowImpl.Object.Resized?.Invoke(clientSize);
+                windowImpl.Object.Resized?.Invoke(clientSize, y);
             });
 
             windowImpl.Setup(x => x.Show(true)).Callback(() =>
@@ -75,7 +76,7 @@ namespace Avalonia.UnitTests
             {
                 clientSize = size.Constrain(s_screenSize);
                 popupImpl.Object.PositionChanged?.Invoke(pos);
-                popupImpl.Object.Resized?.Invoke(clientSize);
+                popupImpl.Object.Resized?.Invoke(clientSize, PlatformResizeReason.Unspecified);
             });
             
             var positioner = new ManagedPopupPositioner(positionerHelper);