chore: Adjust for Uno WinUI

src/Uno.UI.RuntimeTests.Engine.Library/Engine/UI/UnitTestsUIContentHelper.cs

@@ -111,14 +111,8 @@ internal set
 	/// <returns></returns>
 	public static async Task WaitForIdle()
 	{
-#if WINDOWS_WINUI || HAS_UNO_WINUI
-		// This is a wrong implementation. It doesn't really wait for "Idle".
-		await RootElementDispatcher.RunAsync(UnitTestDispatcherCompat.Priority.Low, () => { });
-		await RootElementDispatcher.RunAsync(UnitTestDispatcherCompat.Priority.Low, () => { });
-#else
 		await RootElementDispatcher.RunIdleAsync(_ => { /* Empty to wait for the idle queue to be reached */ });
 		await RootElementDispatcher.RunIdleAsync(_ => { /* Empty to wait for the idle queue to be reached */ });
-#endif
 	}
 
 	/// <summary>

src/Uno.UI.RuntimeTests.Engine.Library/Engine/UI/_Private/UnitTestDispatcherCompat.cs

@@ -47,27 +47,50 @@ public enum Priority { Low = -1, Normal = 0, High = 1 }
 public partial class UnitTestDispatcherCompat
 {
 	private readonly _Impl _impl;
+	private readonly Windows.UI.Core.CoreDispatcher? _dispatcher;
 
+#if WINDOWS_WINUI || HAS_UNO_WINUI
+	public UnitTestDispatcherCompat(_Impl impl, Windows.UI.Core.CoreDispatcher? dispatcher = null)
+	{
+		this._impl = impl;
+		this._dispatcher = dispatcher;
+	}
+#else
 	public UnitTestDispatcherCompat(_Impl impl)
 	{
 		this._impl = impl;
 	}
+#endif
 
-#if !WINDOWS_WINUI && !HAS_UNO_WINUI
 	public Windows.Foundation.IAsyncAction RunIdleAsync(Windows.UI.Core.IdleDispatchedHandler agileCallback)
-		=> _impl.RunIdleAsync(agileCallback);
+	{
+#if !WINDOWS_WINUI && !HAS_UNO_WINUI
+		// Windows UWP and Uno UWP can use CoreDispatcher.RunIdleAsync without issues.
+		return _impl.RunIdleAsync(agileCallback);
+#else
+		// For now, Uno WinUI Dispatcher is non-null.
+		// In the future, this will break and it will be null.
+		if (_dispatcher is not null)
+		{
+			return _dispatcher.RunIdleAsync(agileCallback);
+		}
+
+		// This code path is for Windows WinUI, and "potentially" Uno WinUI in future when the breaking change is taken.
+		// This is a wrong implementation. It doesn't really wait for "Idle".
+		return RunAsync(UnitTestDispatcherCompat.Priority.Low, () => { }).AsAsyncAction();
 #endif
+	}
 
 	public static UnitTestDispatcherCompat From(UIElement x) =>
 #if HAS_UNO_WINUI || WINDOWS_WINUI
-		new UnitTestDispatcherCompat(x.DispatcherQueue);
+		new UnitTestDispatcherCompat(x.DispatcherQueue, x.Dispatcher);
 #else
 		new UnitTestDispatcherCompat(x.Dispatcher);
 #endif
 
 	public static UnitTestDispatcherCompat From(Window x) =>
 #if HAS_UNO_WINUI || WINDOWS_WINUI
-		new UnitTestDispatcherCompat(x.DispatcherQueue);
+		new UnitTestDispatcherCompat(x.DispatcherQueue, x.Dispatcher);
 #else
 		new UnitTestDispatcherCompat(x.Dispatcher);
 #endif