Fix EllipseGeometry clone method

src/Avalonia.Base/Media/EllipseGeometry.cs

@@ -56,6 +56,10 @@ namespace Avalonia.Media
         /// <summary>
         /// Gets or sets a rect that defines the bounds of the ellipse.
         /// </summary>
+        /// <remarks>
+        /// When set, this takes priority over the other properties that define an
+        /// ellipse using a center point and X/Y-axis radii.
+        /// </remarks>
         public Rect Rect
         {
             get => GetValue(RectProperty);
@@ -65,6 +69,10 @@ namespace Avalonia.Media
         /// <summary>
         /// Gets or sets a double that defines the radius in the X-axis of the ellipse.
         /// </summary>
+        /// <remarks>
+        /// In order for this property to be used, <see cref="Rect"/> must not be set
+        /// (equal to the default <see cref="Avalonia.Rect"/> value).
+        /// </remarks>
         public double RadiusX
         {
             get => GetValue(RadiusXProperty);
@@ -74,6 +82,10 @@ namespace Avalonia.Media
         /// <summary>
         /// Gets or sets a double that defines the radius in the Y-axis of the ellipse.
         /// </summary>
+        /// <remarks>
+        /// In order for this property to be used, <see cref="Rect"/> must not be set
+        /// (equal to the default <see cref="Avalonia.Rect"/> value).
+        /// </remarks>
         public double RadiusY
         {
             get => GetValue(RadiusYProperty);
@@ -83,6 +95,10 @@ namespace Avalonia.Media
         /// <summary>
         /// Gets or sets a point that defines the center of the ellipse.
         /// </summary>
+        /// <remarks>
+        /// In order for this property to be used, <see cref="Rect"/> must not be set
+        /// (equal to the default <see cref="Avalonia.Rect"/> value).
+        /// </remarks>
         public Point Center
         {
             get => GetValue(CenterProperty);
@@ -92,7 +108,30 @@ namespace Avalonia.Media
         /// <inheritdoc/>
         public override Geometry Clone()
         {
-            return new EllipseGeometry(Rect);
+            // Note that the ellipse properties are used in two modes:
+            //
+            //  1. Rect-only Mode:
+            //     Directly set the rectangle bounds the ellipse will fill
+            //
+            //  2. Center + Radii Mode:
+            //     Set a center-point and then X/Y-axis radii that are used to
+            //     calculate the rectangle bounds the ellipse will fill.
+            //     This is the only mode supported by WPF.
+            //
+            // Rendering the ellipse will only ever use one of these two modes
+            // based on if the Rect property is set (not equal to default).
+            //
+            // This means it would normally be fine to copy ONLY the Rect property
+            // when it is set. However, while it would render the same, it isn't
+            // a true clone. We want to include all the properties here regardless
+            // of the rendering mode that will eventually be used.
+            return new EllipseGeometry()
+            {
+                Rect = Rect,
+                RadiusX = RadiusX,
+                RadiusY = RadiusY,
+                Center = Center,
+            };
         }
 
         /// <inheritdoc/>