feat: replace mask with pure rectangle to reduce layout calculation

src/Ursa/Controls/Shapes/PureRectangle.cs

@@ -0,0 +1,35 @@
+using Avalonia;
+using Avalonia.Controls.Shapes;
+using Avalonia.Media;
+
+namespace Ursa.Controls.Shapes;
+
+/// <summary>
+/// A rectangle, with no corner radius.
+/// </summary>
+public class PureRectangle: Shape
+{
+    static PureRectangle()
+    {
+        FocusableProperty.OverrideDefaultValue<PureRectangle>(false);
+        AffectsGeometry<PureRectangle>(BoundsProperty);
+    }
+    protected override Geometry? CreateDefiningGeometry()
+    {
+        StreamGeometry geometry = new StreamGeometry();
+        Rect rect = new Rect(this.Bounds.Size).Deflate(this.StrokeThickness / 2.0);
+        using StreamGeometryContext context = geometry.Open();
+        context.BeginFigure(new Point(rect.Left, rect.Top), true);
+        context.LineTo(new Point(rect.Right, rect.Top));
+        context.LineTo(new Point(rect.Right, rect.Bottom));
+        context.LineTo(new Point(rect.Left, rect.Bottom));
+        context.LineTo(new Point(rect.Left, rect.Top));
+        context.EndFigure(true);
+        return geometry;
+    }
+
+    protected override Size MeasureOverride(Size availableSize)
+    {
+        return new Size(this.StrokeThickness, this.StrokeThickness);
+    }
+}