Implement GeometryBoundsHelper

src/Avalonia.Visuals/Rendering/SceneGraph/GeometryBoundsHelper.cs

@@ -0,0 +1,31 @@
+using System;
+using Avalonia.Media;
+using Avalonia.Utilities;
+
+namespace Avalonia.Rendering.SceneGraph;
+
+internal static class GeometryBoundsHelper
+{
+    /// <summary>
+    /// Calculates the bounds of a given geometry with respect to the pens <see cref="IPen.LineCap"/>
+    /// </summary>
+    /// <param name="originalBounds">The calculated bounds without <see cref="IPen.LineCap"/>s</param>
+    /// <param name="pen">The pen with information about the <see cref="IPen.LineCap"/>s</param>
+    /// <returns></returns>
+    public static Rect CalculateBoundsWithLineCaps(this Rect originalBounds, IPen? pen)
+    {
+        if (pen is null || MathUtilities.IsZero(pen.Thickness)) return originalBounds;
+
+        switch (pen.LineCap)
+        {
+            case PenLineCap.Flat:
+                return originalBounds;
+            case PenLineCap.Round:
+                return originalBounds.Inflate(pen.Thickness / 2);
+            case PenLineCap.Square:
+                return originalBounds.Inflate(pen.Thickness);
+            default:
+                throw new ArgumentOutOfRangeException();
+        }
+    }
+}

src/Avalonia.Visuals/Rendering/SceneGraph/GeometryNode.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Rendering.SceneGraph
             IPen? pen,
             IGeometryImpl geometry,
             IDictionary<IVisual, Scene>? childScenes = null)
-            : base(geometry.GetRenderBounds(pen), transform)
+            : base(geometry.GetRenderBounds(pen).CalculateBoundsWithLineCaps(pen), transform)
         {
             Transform = transform;
             Brush = brush?.ToImmutable();