Fix border inner rounded clip

samples/ControlCatalog/MainView.xaml

@@ -34,6 +34,9 @@
       <TabItem Header="Border">
         <pages:BorderPage />
       </TabItem>
+      <TabItem Header="Child Clipping">
+        <pages:ChildClippingPage />
+      </TabItem>
       <TabItem Header="BitmapCache">
         <pages:BitmapCachePage />
       </TabItem>

samples/ControlCatalog/Pages/BorderPage.xaml

@@ -56,6 +56,52 @@
         <Image Source="/Assets/maple-leaf-888807_640.jpg" Stretch="UniformToFill" />
       </Border>
       <TextBlock Text="Border with Clipping" HorizontalAlignment="Center" />
+
+      <StackPanel Spacing="8" Width="360">
+        <TextBlock Classes="h3">Rounded Border Child Clipping</TextBlock>
+
+        <Border Height="72"
+          Width="160"
+          Background="Red"
+          BorderBrush="Green"
+          BorderThickness="2"
+          CornerRadius="16">
+          <Border Width="80"
+            Height="72"
+            Background="Blue"
+            HorizontalAlignment="Right" />
+        </Border>
+        <TextBlock Text="Expected: child overlaps rounded corners" HorizontalAlignment="Center" />
+        <TextBlock Text="No ClipToBounds" HorizontalAlignment="Center" />
+
+        <Border Height="72"
+          Width="160"
+          Background="Red"
+          BorderBrush="Green"
+          BorderThickness="2"
+          CornerRadius="16"
+          ClipToBounds="True">
+          <Border Width="80"
+            Height="72"
+            Background="Blue"
+            HorizontalAlignment="Right" />
+        </Border>
+        <TextBlock Text="Expected: child clipped to inner rounded edge" HorizontalAlignment="Center" />
+        <TextBlock Text="ClipToBounds True" HorizontalAlignment="Center" />
+
+        <Border Height="80"
+          Width="200"
+          Background="LightGray"
+          BorderBrush="Black"
+          BorderThickness="6"
+          CornerRadius="16"
+          ClipToBounds="True">
+          <Border Background="Orange"
+                  HorizontalAlignment="Stretch"
+                  VerticalAlignment="Stretch" />
+        </Border>
+        <TextBlock Text="Thick Border + Rounded Clip" HorizontalAlignment="Center" />
+      </StackPanel>
     </StackPanel>
   </StackPanel>
 </UserControl>

samples/ControlCatalog/Pages/ChildClippingPage.xaml

@@ -0,0 +1,83 @@
+<UserControl x:Class="ControlCatalog.Pages.ChildClippingPage"
+             xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:pages="using:ControlCatalog.Pages"
+             mc:Ignorable="d"
+             d:DesignHeight="800"
+             d:DesignWidth="400">
+  <ScrollViewer>
+    <StackPanel Margin="16" Spacing="16">
+      <TextBlock Classes="h2">Child Clipping</TextBlock>
+      <TextBlock TextWrapping="Wrap">
+        Border and ContentPresenter now clip only their children when ClipToBounds is enabled.
+        The control's own background and border are not clipped. Adorners respect rounded clips too.
+      </TextBlock>
+
+      <StackPanel Spacing="8">
+        <TextBlock Classes="h3">Border: Rounded Child Clip</TextBlock>
+        <Border Height="72"
+                Width="160"
+                Background="Red"
+                BorderBrush="Green"
+                BorderThickness="2"
+                CornerRadius="16"
+                ClipToBounds="True">
+          <Border Width="80"
+                  Height="72"
+                  Background="Blue"
+                  HorizontalAlignment="Right" />
+        </Border>
+        <TextBlock Text="Only the child is clipped; the border/background remain intact" />
+      </StackPanel>
+
+      <StackPanel Spacing="8">
+        <TextBlock Classes="h3">ContentPresenter: Rounded Child Clip</TextBlock>
+        <ContentPresenter Width="160"
+                          Height="72"
+                          Background="Red"
+                          BorderBrush="Green"
+                          BorderThickness="2"
+                          CornerRadius="16"
+                          ClipToBounds="True"
+                          HorizontalContentAlignment="Right">
+          <ContentPresenter.Content>
+            <Border Width="80"
+                    Height="72"
+                    Background="Blue" />
+          </ContentPresenter.Content>
+        </ContentPresenter>
+        <TextBlock Text="Only the child is clipped; the presenter background remains intact" />
+      </StackPanel>
+
+      <StackPanel Spacing="8">
+        <TextBlock Classes="h3">Adorner: Respects Rounded Clip</TextBlock>
+        <Border Width="160"
+                Height="160"
+                Background="Transparent"
+                BorderBrush="Black"
+                BorderThickness="4"
+                CornerRadius="24"
+                ClipToBounds="True">
+          <AdornerLayer.Adorner>
+            <Border Background="DeepSkyBlue"
+                    Opacity="0.6"
+                    AdornerLayer.IsClipEnabled="True" />
+          </AdornerLayer.Adorner>
+        </Border>
+        <TextBlock Text="Adorner is clipped to the rounded bounds of the adorned control" />
+      </StackPanel>
+
+      <StackPanel Spacing="8">
+        <TextBlock Classes="h3">Geometry Child Clip (Layout Clip)</TextBlock>
+        <pages:StarLayoutClipDecorator Width="200"
+                                       Height="200"
+                                       ClipToBounds="False">
+          <Border Background="DodgerBlue" />
+        </pages:StarLayoutClipDecorator>
+        <TextBlock Text="GetLayoutClip affects only children; ClipToBounds is not required" />
+      </StackPanel>
+    </StackPanel>
+  </ScrollViewer>
+</UserControl>

samples/ControlCatalog/Pages/ChildClippingPage.xaml.cs

@@ -0,0 +1,73 @@
+using System;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Media;
+
+namespace ControlCatalog.Pages
+{
+    public partial class ChildClippingPage : UserControl
+    {
+        public ChildClippingPage()
+        {
+            InitializeComponent();
+        }
+    }
+
+    public sealed class StarLayoutClipDecorator : Decorator
+    {
+        private Geometry? _clipGeometry;
+        private Size _clipGeometrySize;
+
+        protected override Geometry? GetLayoutClip(Size layoutSlotSize)
+        {
+            return GetClipGeometry(Bounds.Size);
+        }
+
+        public override void Render(DrawingContext context)
+        {
+            context.FillRectangle(Brushes.LightGray, new Rect(Bounds.Size));
+            base.Render(context);
+        }
+
+        private Geometry GetClipGeometry(Size size)
+        {
+            if (_clipGeometry != null && _clipGeometrySize == size)
+                return _clipGeometry;
+
+            _clipGeometrySize = size;
+            _clipGeometry = BuildStarGeometry(size);
+            return _clipGeometry;
+        }
+
+        private static Geometry BuildStarGeometry(Size size)
+        {
+            var geometry = new StreamGeometry();
+            var center = new Point(size.Width * 0.5, size.Height * 0.5);
+            var outerRadius = Math.Min(size.Width, size.Height) * 0.45;
+            var innerRadius = outerRadius * 0.45;
+
+            using (var ctx = geometry.Open())
+            {
+                var angleStep = Math.PI / 5.0;
+                for (var i = 0; i < 10; i++)
+                {
+                    var radius = (i % 2 == 0) ? outerRadius : innerRadius;
+                    var angle = -Math.PI * 0.5 + angleStep * i;
+                    var point = new Point(
+                        center.X + Math.Cos(angle) * radius,
+                        center.Y + Math.Sin(angle) * radius);
+
+                    if (i == 0)
+                        ctx.BeginFigure(point, true);
+                    else
+                        ctx.LineTo(point);
+                }
+
+                ctx.EndFigure(true);
+            }
+
+            return geometry;
+        }
+    }
+
+}

src/Avalonia.Base/Layout/Layoutable.cs

@@ -429,6 +429,7 @@ public void Arrange(Rect rect)
                 Logger.TryGet(LogEventLevel.Verbose, LogArea.Layout)?.Log(this, "Arrange to {Rect} ", rect);
 
                 IsArrangeValid = true;
+                SetLayoutSlotSize(rect.Size);
                 ArrangeCore(rect);
                 _previousArrange = rect;
             }

src/Avalonia.Base/Rendering/Composition/CompositionDrawListVisual.cs

@@ -1,3 +1,4 @@
+using Avalonia.Platform;
 using Avalonia.Rendering.Composition.Drawing;
 using Avalonia.Rendering.Composition.Server;
 using Avalonia.Rendering.Composition.Transport;
@@ -16,6 +17,10 @@ internal class CompositionDrawListVisual : CompositionContainerVisual
 
     private bool _drawListChanged;
     private CompositionRenderData? _drawList;
+    private bool _childClipChanged;
+    private bool _hasChildClip;
+    private RoundedRect _childClip;
+    private IGeometryImpl? _childClipGeometry;
 
     /// <summary>
     /// The list of drawing commands
@@ -36,6 +41,30 @@ public CompositionRenderData? DrawList
         }
     }
 
+    internal void SetChildClip(RoundedRect clip, IGeometryImpl? geometryClip)
+    {
+        if (_hasChildClip && _childClip.Equals(clip) && ReferenceEquals(_childClipGeometry, geometryClip))
+            return;
+
+        _hasChildClip = true;
+        _childClip = clip;
+        _childClipGeometry = geometryClip;
+        _childClipChanged = true;
+        RegisterForSerialization();
+    }
+
+    internal void ClearChildClip()
+    {
+        if (!_hasChildClip && _childClipGeometry == null)
+            return;
+
+        _hasChildClip = false;
+        _childClip = default;
+        _childClipGeometry = null;
+        _childClipChanged = true;
+        RegisterForSerialization();
+    }
+
     private protected override void SerializeChangesCore(BatchStreamWriter writer)
     {
         writer.Write((byte)(_drawListChanged ? 1 : 0));
@@ -44,6 +73,14 @@ private protected override void SerializeChangesCore(BatchStreamWriter writer)
             writer.WriteObject(DrawList?.Server);
             _drawListChanged = false;
         }
+        writer.Write((byte)(_childClipChanged ? 1 : 0));
+        if (_childClipChanged)
+        {
+            writer.Write(_hasChildClip);
+            writer.Write(_childClip);
+            writer.WriteObject(_childClipGeometry);
+            _childClipChanged = false;
+        }
         base.SerializeChangesCore(writer);
     }
 

src/Avalonia.Base/Rendering/Composition/CompositionTarget.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Numerics;
+using Avalonia.Media;
 using Avalonia.Collections.Pooled;
 using Avalonia.VisualTree;
 
@@ -71,16 +72,27 @@ void HitTestCore(CompositionVisual visual, Point parentPoint, PooledList<Composi
                 return;
 
             var point = parentPoint.Transform(invMatrix);
-
-            if (visual.ClipToBounds
-                && (point.X < 0 || point.Y < 0 || point.X > visual.Size.X || point.Y > visual.Size.Y))
+            var allowChildren = true;
+            if (visual is CompositionDrawListVisual { Visual: Visual childClipProvider }
+                && childClipProvider.TryGetChildClip(out var childClip, out var childClipGeometry))
+            {
+                var clipContains = childClipGeometry == null
+                    ? childClip.ContainsExclusive(point)
+                    : childClipGeometry.FillContains(point);
+                if (!clipContains)
+                    allowChildren = false;
+            }
+            else if (visual.ClipToBounds &&
+                     (point.X < 0 || point.Y < 0 || point.X > visual.Size.X || point.Y > visual.Size.Y))
+            {
                 return;
+            }
 
             if (visual.Clip?.FillContains(point) == false)
                 return;
 
             // Inspect children
-            if (visual is CompositionContainerVisual cv)
+            if (allowChildren && visual is CompositionContainerVisual cv)
                 for (var c = cv.Children.Count - 1; c >= 0; c--)
                 {
                     var ch = cv.Children[c];

src/Avalonia.Base/Rendering/Composition/Server/ServerCompositionDrawListVisual.cs

@@ -19,6 +19,9 @@ internal class ServerCompositionDrawListVisual : ServerCompositionContainerVisua
     public readonly Visual UiVisual;
 #endif
     private ServerCompositionRenderData? _renderCommands;
+    private bool _hasChildClip;
+    private RoundedRect _childClip;
+    private IGeometryImpl? _childClipGeometry;
 
     public ServerCompositionDrawListVisual(ServerCompositor compositor, Visual v) : base(compositor)
     {
@@ -29,6 +32,10 @@ public ServerCompositionDrawListVisual(ServerCompositor compositor, Visual v) :
 
     public override LtrbRect? ComputeOwnContentBounds() => _renderCommands?.Bounds;
 
+    public bool HasChildClip => _hasChildClip;
+    public RoundedRect ChildClip => _childClip;
+    public IGeometryImpl? ChildClipGeometry => _childClipGeometry;
+
     protected override void DeserializeChangesCore(BatchStreamReader reader, TimeSpan committedAt)
     {
         if (reader.Read<byte>() == 1)
@@ -38,6 +45,13 @@ protected override void DeserializeChangesCore(BatchStreamReader reader, TimeSpa
             _renderCommands?.AddObserver(this);
             InvalidateContent();
         }
+        if (reader.Read<byte>() == 1)
+        {
+            _hasChildClip = reader.Read<bool>();
+            _childClip = reader.Read<RoundedRect>();
+            _childClipGeometry = reader.ReadObject<IGeometryImpl?>();
+            InvalidateContent();
+        }
         base.DeserializeChangesCore(reader, committedAt);
     }
 

src/Avalonia.Base/Rendering/Composition/Server/ServerCompositionVisual/ServerCompositionVisual.Adorners.cs

@@ -92,7 +92,7 @@ private bool AdornerLayer_WalkAdornerParentClipRecursive(ServerCompositionVisual
 
             if (visual.ClipToBounds)
             {
-                _canvas.PushClip(new Rect(0, 0, visual.Size.X, visual.Size.Y));
+                visual.PushClipToBounds(_canvas);
                 _adornerPushedClipStack!.Push((int)Op.PopClip);
             }