Introduced "forced" CSD mode without app opting in

api/Avalonia.nupkg.xml

@@ -2395,6 +2395,12 @@
     <Left>baseline/Avalonia/lib/net10.0/Avalonia.Controls.dll</Left>
     <Right>current/Avalonia/lib/net10.0/Avalonia.Controls.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>M:Avalonia.Controls.Window.ExtendClientAreaToDecorationsChanged(System.Boolean)</Target>
+    <Left>baseline/Avalonia/lib/net10.0/Avalonia.Controls.dll</Left>
+    <Right>current/Avalonia/lib/net10.0/Avalonia.Controls.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>M:Avalonia.Controls.Window.get_ExtendClientAreaChromeHints</Target>
@@ -4063,6 +4069,12 @@
     <Left>baseline/Avalonia/lib/net8.0/Avalonia.Controls.dll</Left>
     <Right>current/Avalonia/lib/net8.0/Avalonia.Controls.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>M:Avalonia.Controls.Window.ExtendClientAreaToDecorationsChanged(System.Boolean)</Target>
+    <Left>baseline/Avalonia/lib/net8.0/Avalonia.Controls.dll</Left>
+    <Right>current/Avalonia/lib/net8.0/Avalonia.Controls.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>M:Avalonia.Controls.Window.get_ExtendClientAreaChromeHints</Target>

src/Avalonia.Controls/PresentationSource/PresentationSource.RenderRoot.cs

@@ -1,4 +1,3 @@
-using System;
 using Avalonia.Input;
 using Avalonia.Rendering;
 using Avalonia.Rendering.Composition;
@@ -7,7 +6,6 @@ namespace Avalonia.Controls;
 
 internal partial class PresentationSource
 {
-    private readonly Func<Size> _clientSizeProvider;
     public CompositingRenderer Renderer { get; }
     IRenderer IPresentationSource.Renderer => Renderer;
     Visual IPresentationSource.RootVisual => RootVisual;
@@ -16,7 +14,7 @@ internal partial class PresentationSource
     public IHitTester? HitTesterOverride { get; set; }
 
     public double RenderScaling => PlatformImpl?.RenderScaling ?? 1;
-    public Size ClientSize => _clientSizeProvider();
+    public Size ClientSize => PlatformImpl?.ClientSize ?? default;
 
     public void SceneInvalidated(object? sender, SceneInvalidatedEventArgs sceneInvalidatedEventArgs)
     {

src/Avalonia.Controls/PresentationSource/PresentationSource.cs

@@ -23,10 +23,8 @@ internal partial class PresentationSource : IPresentationSource, IInputRoot, IDi
 
     public PresentationSource(InputElement rootVisual, InputElement defaultFocusVisual,
         ITopLevelImpl platformImpl,
-        IAvaloniaDependencyResolver dependencyResolver, Func<Size> clientSizeProvider)
+        IAvaloniaDependencyResolver dependencyResolver)
     {
-        _clientSizeProvider = clientSizeProvider;
-
         PlatformImpl = platformImpl;
 
 

src/Avalonia.Controls/TopLevel.cs

@@ -212,7 +212,7 @@ internal TopLevel(ITopLevelImpl impl, IAvaloniaDependencyResolver? dependencyRes
             LogicalChildren.Add(hostVisual);
 
             _source = new PresentationSource(hostVisual, this,
-                impl, dependencyResolver, () => ClientSize);
+                impl, dependencyResolver);
             _source.Renderer.SceneInvalidated += SceneInvalidated;
 
             _scaling = ValidateScaling(impl.RenderScaling);

src/Avalonia.Controls/TopLevelHost.cs

@@ -3,6 +3,7 @@
 using Avalonia.Automation.Peers;
 using Avalonia.Controls.Chrome;
 using Avalonia.Input;
+using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Reactive;
 
@@ -14,6 +15,8 @@ namespace Avalonia.Controls;
 /// </summary>
 internal partial class TopLevelHost : Control
 {
+    private Thickness _decorationInset;
+
     static TopLevelHost()
     {
         KeyboardNavigation.TabNavigationProperty.OverrideDefaultValue<TopLevelHost>(KeyboardNavigationMode.Cycle);
@@ -25,5 +28,89 @@ public TopLevelHost(TopLevel tl)
         VisualChildren.Add(tl);
     }
 
+    /// <summary>
+    /// Gets or sets the decoration inset applied to the TopLevel child in forced decoration mode.
+    /// When non-zero, the TopLevel is measured and arranged within the inset area while
+    /// decoration layers use the full available size.
+    /// </summary>
+    internal Thickness DecorationInset
+    {
+        get => _decorationInset;
+        set
+        {
+            if (_decorationInset == value)
+                return;
+            _decorationInset = value;
+            InvalidateMeasure();
+        }
+    }
+
+    protected override Size MeasureOverride(Size availableSize)
+    {
+        var inset = _decorationInset;
+        var hasInset = inset != default;
+        var desiredSize = default(Size);
+
+        foreach (var child in VisualChildren)
+        {
+            if (child is Layoutable l)
+            {
+                if (hasInset && ReferenceEquals(child, _topLevel))
+                {
+                    // In forced mode, measure the TopLevel with reduced size
+                    var contentSize = new Size(
+                        Math.Max(0, availableSize.Width - inset.Left - inset.Right),
+                        Math.Max(0, availableSize.Height - inset.Top - inset.Bottom));
+                    l.Measure(contentSize);
+
+                    // Add inset back so TopLevelHost's desired size represents the full frame.
+                    // This ensures ArrangeOverride receives the full frame size and can correctly
+                    // position the TopLevel within the inset area.
+                    desiredSize = new Size(
+                        Math.Max(desiredSize.Width, l.DesiredSize.Width + inset.Left + inset.Right),
+                        Math.Max(desiredSize.Height, l.DesiredSize.Height + inset.Top + inset.Bottom));
+                }
+                else
+                {
+                    l.Measure(availableSize);
+
+                    desiredSize = new Size(
+                        Math.Max(desiredSize.Width, l.DesiredSize.Width),
+                        Math.Max(desiredSize.Height, l.DesiredSize.Height));
+                }
+            }
+        }
+
+        return desiredSize;
+    }
+
+    protected override Size ArrangeOverride(Size finalSize)
+    {
+        var inset = _decorationInset;
+        var hasInset = inset != default;
+
+        foreach (var child in VisualChildren)
+        {
+            if (child is Layoutable l)
+            {
+                if (hasInset && ReferenceEquals(child, _topLevel))
+                {
+                    // In forced mode, arrange the TopLevel within the inset area
+                    var contentSize = new Size(
+                        Math.Max(0, finalSize.Width - inset.Left - inset.Right),
+                        Math.Max(0, finalSize.Height - inset.Top - inset.Bottom));
+
+                    l.Arrange(new Rect(inset.Left, inset.Top, contentSize.Width, contentSize.Height));
+                }
+                else
+                {
+                    l.Arrange(new Rect(finalSize));
+                }
+            }
+        }
+
+        return finalSize;
+    }
+
     protected override bool BypassFlowDirectionPolicies => true;
 }

src/Avalonia.Controls/Window.cs

@@ -98,6 +98,7 @@ public class Window : WindowBase, IFocusScope
         private Thickness _offScreenMargin;
         private bool _canHandleResized = false;
         private Size _arrangeBounds;
+        private bool _isForcedDecorationMode;
 
         /// <summary>
         /// Defines the <see cref="SizeToContent"/> property.
@@ -249,7 +250,10 @@ public Window(IWindowImpl impl)
             impl.WindowStateChanged = HandleWindowStateChanged;
             _maxPlatformClientSize = PlatformImpl?.MaxAutoSizeHint ?? default(Size);
             impl.ExtendClientAreaToDecorationsChanged = ExtendClientAreaToDecorationsChanged;
-            this.GetObservable(ClientSizeProperty).Skip(1).Subscribe(x => PlatformImpl?.Resize(x, WindowResizeReason.Application));
+            this.GetObservable(ClientSizeProperty).Skip(1).Subscribe(x =>
+            {
+                ResizePlatformImpl(x, WindowResizeReason.Application);
+            });
 
             CreatePlatformImplBinding(TitleProperty, title => PlatformImpl!.SetTitle(title));
             CreatePlatformImplBinding(IconProperty, SetEffectiveIcon);
@@ -668,7 +672,7 @@ private void HandleWindowStateChanged(WindowState state)
             UpdateDrawnDecorationParts();
         }
 
-        protected virtual void ExtendClientAreaToDecorationsChanged(bool isExtended)
+        private void ExtendClientAreaToDecorationsChanged(bool isExtended)
         {
             IsExtendedIntoWindowDecorations = isExtended;
             OffScreenMargin = PlatformImpl?.OffScreenMargin ?? default;
@@ -679,6 +683,10 @@ protected virtual void ExtendClientAreaToDecorationsChanged(bool isExtended)
         private void UpdateDrawnDecorations()
         {
             var parts = ComputeDecorationParts();
+
+            // Detect forced mode: platform needs managed decorations but app hasn't opted in
+            _isForcedDecorationMode = parts != null && !IsExtendedIntoWindowDecorations;
+
             TopLevelHost.UpdateDrawnDecorations(parts, WindowState);
 
             if (parts != null)
@@ -753,7 +761,9 @@ private void UpdateDrawnDecorationMargins()
             var decorations = TopLevelHost.Decorations;
             if (decorations == null)
             {
+                // Only use platform margins if drawn decorations are not active
                 WindowDecorationMargin = PlatformImpl?.ExtendedMargins ?? default;
+                TopLevelHost.DecorationInset = default;
                 return;
             }
 
@@ -764,11 +774,25 @@ private void UpdateDrawnDecorationMargins()
                 ? decorations.FrameThickness : default;
             var shadow = parts.HasFlag(Chrome.DrawnWindowDecorationParts.Shadow)
                 ? decorations.ShadowThickness : default;
-            WindowDecorationMargin = new Thickness(
+            var margin = new Thickness(
                 frame.Left + shadow.Left,
                 titleBarHeight + frame.Top + shadow.Top,
                 frame.Right + shadow.Right,
                 frame.Bottom + shadow.Bottom);
+
+            if (_isForcedDecorationMode)
+            {
+                // In forced mode, app is unaware of decorations.
+                // TopLevelHost insets the Window child; WindowDecorationMargin stays zero.
+                WindowDecorationMargin = default;
+                TopLevelHost.DecorationInset = margin;
+            }
+            else
+            {
+                // In extended mode, app handles the margin itself.
+                WindowDecorationMargin = margin;
+                TopLevelHost.DecorationInset = default;
+            }
         }
 
         private void OnTitleBarHeightHintChanged()
@@ -933,6 +957,15 @@ private void EnsureParentStateBeforeShow(Window owner)
                 // Enable drawn decorations before layout so margins are computed
                 UpdateDrawnDecorations();
 
+                // In forced mode, adjust ClientSize to reflect usable content area
+                if (_isForcedDecorationMode)
+                {
+                    var inset = TopLevelHost.DecorationInset;
+                    ClientSize = new Size(
+                        Math.Max(0, ClientSize.Width - inset.Left - inset.Right),
+                        Math.Max(0, ClientSize.Height - inset.Top - inset.Bottom));
+                }
+
                 _shown = true;
                 IsVisible = true;
 
@@ -978,10 +1011,18 @@ private void EnsureParentStateBeforeShow(Window owner)
 
                 DesktopScalingOverride = null;
 
-                if (clientSizeChanged || ClientSize != PlatformImpl?.ClientSize)
+                // In forced mode, compare against adjusted platform size
+                var platformClientSize = PlatformImpl?.ClientSize ?? default;
+                var comparableClientSize = _isForcedDecorationMode
+                    ? new Size(
+                        Math.Max(0, platformClientSize.Width - TopLevelHost.DecorationInset.Left - TopLevelHost.DecorationInset.Right),
+                        Math.Max(0, platformClientSize.Height - TopLevelHost.DecorationInset.Top - TopLevelHost.DecorationInset.Bottom))
+                    : platformClientSize;
+
+                if (clientSizeChanged || ClientSize != comparableClientSize)
                 {
                     // Previously it was called before ExecuteInitialLayoutPass
-                    PlatformImpl?.Resize(ClientSize, WindowResizeReason.Layout);
+                    ResizePlatformImpl(ClientSize, WindowResizeReason.Layout);
 
                     // we do not want PlatformImpl?.Resize to trigger HandleResized yet because it will set Width and Height.
                     // So perform some important actions from HandleResized
@@ -1037,6 +1078,22 @@ private void EnsureParentStateBeforeShow(Window owner)
             }
         }
 
+        private void ResizePlatformImpl(Size size, WindowResizeReason reason)
+        {
+            // In forced mode, add decoration inset so platform gets full frame size
+            if (_isForcedDecorationMode)
+            {
+                var inset = TopLevelHost.DecorationInset;
+                size = new Size(
+                    size.Width + inset.Left + inset.Right,
+                    size.Height + inset.Top + inset.Bottom);
+                if (PlatformImpl?.ClientSize != size)
+                    PlatformImpl?.Resize(size, reason);
+            }
+            else
+                PlatformImpl?.Resize(size, reason);
+        }
+
         /// <summary>
         /// Shows the window as a dialog.
         /// </summary>
@@ -1272,6 +1329,14 @@ protected override Size MeasureOverride(Size availableSize)
         {
             var sizeToContent = SizeToContent;
             var clientSize = ClientSize;
+            if (_isForcedDecorationMode)
+            {
+                clientSize = PlatformImpl?.ClientSize ?? clientSize;
+                var inset = TopLevelHost.DecorationInset;
+                clientSize = new Size(
+                    Math.Max(0, clientSize.Width - inset.Left - inset.Right),
+                    Math.Max(0, clientSize.Height - inset.Top - inset.Bottom));
+            }
             var maxAutoSize = PlatformImpl?.MaxAutoSizeHint ?? Size.Infinity;
             var useAutoWidth = sizeToContent.HasAllFlags(SizeToContent.Width);
             var useAutoHeight = sizeToContent.HasAllFlags(SizeToContent.Height);
@@ -1332,7 +1397,9 @@ private protected sealed override Size ArrangeSetBounds(Size size)
         {
             _arrangeBounds = size;
             if (_canHandleResized)
-                PlatformImpl?.Resize(size, WindowResizeReason.Layout);
+            {
+                ResizePlatformImpl(size, WindowResizeReason.Layout);
+            }
             return ClientSize;
         }
 
@@ -1350,6 +1417,16 @@ private protected sealed override void HandleClosed()
         /// <inheritdoc/>
         internal override void HandleResized(Size clientSize, WindowResizeReason reason)
         {
+            // In forced decoration mode, the platform's clientSize includes decoration area.
+            // Subtract the decoration inset so Window.ClientSize reflects the usable content area.
+            if (_isForcedDecorationMode)
+            {
+                var inset = TopLevelHost.DecorationInset;
+                clientSize = new Size(
+                    Math.Max(0, clientSize.Width - inset.Left - inset.Right),
+                    Math.Max(0, clientSize.Height - inset.Top - inset.Bottom));
+            }
+
             if (_canHandleResized && (ClientSize != clientSize || double.IsNaN(Width) || double.IsNaN(Height)))
             {
                 var sizeToContent = SizeToContent;

src/Avalonia.Controls/WindowBase.cs

@@ -304,7 +304,7 @@ protected override void ArrangeCore(Rect finalRect)
         {
             var constraint = ArrangeSetBounds(finalRect.Size);
             var arrangeSize = ArrangeOverride(constraint);
-            Bounds = new Rect(arrangeSize);
+            Bounds = new Rect(finalRect.Position, arrangeSize);
         }
 
         /// <summary>