fix: [Skia] Fix layout when RevealBrush is used

Fall back on FallbackColor when RevealBrush is assigned, instead of trying to use unimplemented CompositionBrush property which throws an exception.

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Media/Given_RevealBrush.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Windows.UI;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Media;
+using static Private.Infrastructure.TestServices;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Media
+{
+	[TestClass]
+	[RunsOnUIThread]
+	public class Given_RevealBrush
+	{
+		[TestMethod]
+		public async Task When_RevealBrush_Assigned()
+		{
+			// Basic smoke test - RevealBrush is currently unimplemented, but it shouldn't break the layout
+
+			var siblingBorder = new Border { Width = 43, Height = 22 };
+			var gridWithRevealBackground = new Grid
+			{
+				Width = 57,
+				Height = 34,
+				Background = new RevealBackgroundBrush() { Color = Colors.Brown, FallbackColor = Colors.Brown },
+			};
+			var parentSP = new StackPanel
+			{
+				Children =
+				{
+					gridWithRevealBackground,
+					siblingBorder
+				}
+			};
+
+			WindowHelper.WindowContent = parentSP;
+			await WindowHelper.WaitForLoaded(parentSP);
+
+			await WindowHelper.WaitForEqual(57, () => gridWithRevealBackground.ActualWidth);
+			Assert.AreEqual(34, gridWithRevealBackground.ActualHeight);
+
+			Assert.AreEqual(43, siblingBorder.ActualWidth);
+			Assert.AreEqual(22, siblingBorder.ActualHeight);
+		}
+	}
+}

src/Uno.UI/UI/Xaml/Media/Brush.skia.cs

@@ -107,9 +107,9 @@ internal static IDisposable AssignAndObserveBrush(Brush brush, Compositor compos
 			{
 				return AssignAndObserveAcrylicBrush(acrylicBrush, compositor, brushSetter);
 			}
-			else if (brush is XamlCompositionBrushBase xamlCompositionBrushBase)
+			else if (brush is XamlCompositionBrushBase unimplementedCompositionBrush)
 			{
-				return AssignAndObserveXamlCompositionBrush(xamlCompositionBrushBase, compositor, brushSetter);
+				return AssignAndObserveXamlCompositionBrush(unimplementedCompositionBrush, compositor, brushSetter);
 			}
 			else
 			{
@@ -245,17 +245,27 @@ private static IDisposable AssignAndObserveAcrylicBrush(AcrylicBrush acrylicBrus
 			return disposables;
 		}
 
+		/// <summary>
+		/// Apply fallback colour for unimplemented <see cref="XamlCompositionBrushBase"/> types. For implemented types a more specific method
+		/// should be supplied.
+		/// </summary>
 		private static IDisposable AssignAndObserveXamlCompositionBrush(XamlCompositionBrushBase brush, Compositor compositor, BrushSetterHandler brushSetter)
 		{
 			var disposables = new CompositeDisposable();
 
-			var compositionBrush = brush.CompositionBrush;
+			var compositionBrush = compositor.CreateColorBrush(brush.FallbackColorWithOpacity);
 
-			//brush.RegisterDisposablePropertyChangedCallback(
-			//	XamlCompositionBrushBase.CompositionBrushProperty,
-			//	(s, e) => brushSetter(((CompositionBrush)e.NewValue))
-			//)
-			//.DisposeWith(disposables);
+			brush.RegisterDisposablePropertyChangedCallback(
+				AcrylicBrush.FallbackColorProperty,
+				(s, colorArg) => compositionBrush.Color = brush.FallbackColorWithOpacity
+			)
+			.DisposeWith(disposables);
+
+			brush.RegisterDisposablePropertyChangedCallback(
+				AcrylicBrush.OpacityProperty,
+				(s, colorArg) => compositionBrush.Color = brush.FallbackColorWithOpacity
+			)
+			.DisposeWith(disposables);
 
 			brushSetter(compositionBrush);