[Windows] Fix Border corners clipping issue

src/Controls/samples/Controls.Sample/Pages/Core/BorderGalleries/BorderClipPlayground.xaml

@@ -17,13 +17,14 @@
     <Grid
         Padding="12">
         <Grid.RowDefinitions>
-            <RowDefinition Height="80" />
+            <RowDefinition Height="110" />
             <RowDefinition Height="*"/>
         </Grid.RowDefinitions>
 
-        <Border x:Name="BorderView" Margin="5"
-                Stroke="Red" Padding="0"
-                HorizontalOptions="Center" HeightRequest="100" WidthRequest="100">
+        <Border 
+            x:Name="BorderView" Margin="5"
+            Stroke="Red" Padding="0"
+            HorizontalOptions="Center" HeightRequest="100" WidthRequest="100">
             <Image x:Name="BorderContent" Aspect="AspectFill" Source="oasis.jpg" />
         </Border>
 

src/Controls/src/Core/Border/Border.cs

@@ -314,7 +314,7 @@ void OnStrokeDashArrayChanged(object? sender, NotifyCollectionChangedEventArgs e
 
 		void UpdateStrokeShape()
 		{
-			if (StrokeShape is Shape strokeShape)
+			if (StrokeShape is Shape strokeShape && StrokeThickness == 0)
 			{
 				strokeShape.StrokeThickness = StrokeThickness;
 			}

src/Controls/tests/DeviceTests/Elements/Border/BorderTests.Windows.cs

@@ -0,0 +1,58 @@
+using Xunit;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Graphics;
+using Microsoft.Maui.Controls.Shapes;
+using Microsoft.Maui.Handlers;
+using Microsoft.UI.Xaml.Hosting;
+using Microsoft.Maui.Platform;
+using System.Threading.Tasks;
+using Microsoft.UI.Composition;
+
+namespace Microsoft.Maui.DeviceTests
+{
+	public partial class BorderTests : ControlsHandlerTestBase
+	{
+		[Theory(DisplayName = "Inner CornerRadius Initializes Correctly")]
+		[InlineData(0)]
+		[InlineData(12)]
+		[InlineData(24)]
+		public async Task InnerCornerRadiusInitializesCorrectly(int cornerRadius)
+		{
+			SetupBuilder();
+
+			var expected = Colors.Red;
+
+			var border = new Border()
+			{
+				Content = new Label { Text = "Background", TextColor = Colors.White },
+				StrokeShape = new RoundRectangle { CornerRadius = cornerRadius },
+				Background = new SolidPaint(expected),
+				StrokeThickness = 0,
+				HeightRequest = 100,
+				WidthRequest = 300
+			};
+
+			await AttachAndRun(border, (handler) =>
+			{
+				var contentPanel = GetNativeBorder(handler as BorderHandler);
+				var content = contentPanel.Content;
+				var visual = ElementCompositionPreview.GetElementVisual(content);
+
+				var clip = visual.Clip as CompositionGeometricClip;
+				Assert.NotNull(clip);
+
+				var geometry = clip.Geometry as CompositionPathGeometry;
+				var path = geometry.Path;
+				Assert.NotNull(path);
+
+				Assert.True(contentPanel.IsInnerPath);
+			});
+
+			await AssertColorAtPoint(border, expected, typeof(BorderHandler), cornerRadius, cornerRadius);
+		}
+
+		ContentPanel GetNativeBorder(BorderHandler borderHandler) =>
+			borderHandler.PlatformView;
+
+	}
+}

src/Controls/tests/DeviceTests/Elements/Border/BorderTests.cs

@@ -18,8 +18,9 @@ void SetupBuilder()
 			{
 				builder.ConfigureMauiHandlers(handlers =>
 				{
-					handlers.AddHandler<Label, LabelHandler>();
 					handlers.AddHandler<Border, BorderHandler>();
+					handlers.AddHandler<Grid, LayoutHandler>();
+					handlers.AddHandler<Label, LabelHandler>();
 				});
 			});
 		}

src/Core/src/Platform/Windows/ContentPanel.cs

@@ -15,7 +15,7 @@ namespace Microsoft.Maui.Platform
 	public class ContentPanel : MauiPanel
 	{
 		readonly Path? _borderPath;
-		IShape? _borderShape;
+		IBorderStroke? _borderStroke;
 		FrameworkElement? _content;
 
 		internal Path? BorderPath => _borderPath;
@@ -30,6 +30,8 @@ internal FrameworkElement? Content
 			}
 		}
 
+		internal bool IsInnerPath { get; private set; }
+
 		protected override global::Windows.Foundation.Size ArrangeOverride(global::Windows.Foundation.Size finalSize)
 		{
 			var actual = base.ArrangeOverride(finalSize);
@@ -52,9 +54,15 @@ void ContentPanelSizeChanged(object sender, UI.Xaml.SizeChangedEventArgs e)
 			if (_borderPath == null)
 				return;
 
-			_borderPath.UpdatePath(_borderShape, ActualWidth, ActualHeight);
+			var width = e.NewSize.Width;
+			var height = e.NewSize.Height;
+
+			if (width <= 0 || height <= 0)
+				return;
+
+			_borderPath.UpdatePath(_borderStroke?.Shape, width, height);
 			UpdateContent();
-			UpdateClip(_borderShape);
+			UpdateClip(_borderStroke?.Shape, width, height);
 		}
 
 		internal void EnsureBorderPath()
@@ -73,16 +81,40 @@ public void UpdateBackground(Paint? background)
 			_borderPath.UpdateBackground(background);
 		}
 
+		[Obsolete("Use Microsoft.Maui.Platform.UpdateBorderStroke instead")]
 		public void UpdateBorderShape(IShape borderShape)
 		{
-			_borderShape = borderShape;
+			UpdateBorder(borderShape);
+		}
 
-			if (_borderPath == null)
+		internal void UpdateBorderStroke(IBorderStroke borderStroke)
+		{
+			if (borderStroke is null)
+				return;
+
+			_borderStroke = borderStroke;
+
+			if (_borderStroke is null)
+				return;
+
+			UpdateBorder(_borderStroke.Shape);
+		}
+
+		void UpdateBorder(IShape? strokeShape)
+		{
+			if (strokeShape is null || _borderPath is null)
 				return;
 
-			_borderPath.UpdateBorderShape(_borderShape, ActualWidth, ActualHeight);
+			_borderPath.UpdateBorderShape(strokeShape, ActualWidth, ActualHeight);
 			UpdateContent();
-			UpdateClip(_borderShape);
+
+			var width = ActualWidth;
+			var height = ActualHeight;
+
+			if (width <= 0 || height <= 0)
+				return;
+
+			UpdateClip(strokeShape, width, height);
 		}
 
 		void AddContent(FrameworkElement? content)
@@ -104,27 +136,38 @@ void UpdateContent()
 			Content.RenderTransform = new TranslateTransform() { X = -strokeThickness, Y = -strokeThickness };
 		}
 
-		void UpdateClip(IShape? borderShape)
+		void UpdateClip(IShape? borderShape, double width, double height)
 		{
-			if (Content == null)
+			if (Content is null)
 				return;
 
-			var clipGeometry = borderShape;
-
-			if (clipGeometry == null)
+			if (height <= 0 && width <= 0)
 				return;
 
-			double width = ActualWidth;
-			double height = ActualHeight;
+			var clipGeometry = borderShape;
 
-			if (height <= 0 && width <= 0)
+			if (clipGeometry is null)
 				return;
 
 			var visual = ElementCompositionPreview.GetElementVisual(Content);
 			var compositor = visual.Compositor;
 
+
 			var pathSize = new Graphics.Rect(0, 0, width, height);
-			var clipPath = clipGeometry.PathForBounds(pathSize);
+			PathF? clipPath;
+
+			if (clipGeometry is IRoundRectangle roundedRectangle)
+			{
+				float strokeThickness = (float)(_borderPath?.StrokeThickness ?? 0);
+				clipPath = roundedRectangle.InnerPathForBounds(pathSize, strokeThickness / 2);
+				IsInnerPath = true;
+			}
+			else
+			{
+				clipPath = clipGeometry.PathForBounds(pathSize);
+				IsInnerPath = false;
+			}
+
 			var device = CanvasDevice.GetSharedDevice();
 			var geometry = clipPath.AsPath(device);
 

src/Core/src/Platform/Windows/StrokeExtensions.cs

@@ -4,12 +4,10 @@ public static class StrokeExtensions
 	{
 		public static void UpdateStrokeShape(this ContentPanel platformView, IBorderStroke border)
 		{
-			var shape = border.Shape;
-
-			if (shape == null)
+			if (border is null)
 				return;
 
-			platformView.UpdateBorderShape(shape);
+			platformView.UpdateBorderStroke(border);
 		}
 
 		public static void UpdateStroke(this ContentPanel platformView, IBorderStroke border)

src/Core/tests/DeviceTests/Handlers/Border/BorderHandlerTests.Windows.cs

@@ -1,7 +1,4 @@
-using System;
-using System.Threading.Tasks;
-
-namespace Microsoft.Maui.DeviceTests
+namespace Microsoft.Maui.DeviceTests
 {
 	public partial class BorderHandlerTests
 	{