fix: AppBar and Flyout base fixes for Uno Islands

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Popups/Given_MessageDialog.cs

@@ -68,6 +68,12 @@ public void Should_Not_Close_Open_ContentDialogs()
 		[RunsOnUIThread]
 		public async Task When_Cancel_Then_CloseDialog()
 		{
+			if (WindowHelper.IsXamlIsland)
+			{
+				// MessageDialog is not supported in Uno Islands.
+				return;
+			}
+
 			var messageDialog = new MessageDialog("When_Cancel_Then_CloseDialog");
 			var asyncOperation = messageDialog.ShowAsync();
 

src/Uno.UI.RuntimeTests/Tests/Windows_UI_ViewManagement_ApplicationView/Given_Control.cs

@@ -19,6 +19,11 @@ public class Given_ApplicationView
 		[TestMethod]
 		public void When_StartupTitle_Is_Defined()
 		{
+			if (TestServices.WindowHelper.IsXamlIsland)
+			{
+				return;
+			}
+
 			Assert.AreEqual(Windows.ApplicationModel.Package.Current.DisplayName, StartupTitle);
 		}
 #endif

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_CalendarDatePicker.cs

@@ -23,6 +23,7 @@ public async Task TestCalendarPanelSize()
 
 		var root = (Grid)SUT.FindName("Root");
 		var flyout = (Flyout)FlyoutBase.GetAttachedFlyout(root);
+		flyout.XamlRoot = SUT.XamlRoot;
 		flyout.Open();
 
 		await WindowHelper.WaitForIdle();

src/Uno.UI/UI/Xaml/Controls/AppBar/AppBar.Partial.cs

@@ -1160,7 +1160,7 @@ protected bool GetShouldOpenUp()
 
 					if (WinUICoreServices.Instance.InitializationType == InitializationType.IslandsOnly)
 					{
-						layoutBounds = XamlRoot!.Bounds;
+						layoutBounds = XamlRoot?.Bounds ?? new();
 					}
 
 					shouldOpenUp = offsetFromRootOpenedUp.Y >= layoutBounds.Y;

src/Uno.UI/UI/Xaml/Controls/Flyout/FlyoutBase.cs

@@ -466,6 +466,11 @@ protected internal virtual void Open()
 			SetPopupPosition(Target, PopupPositionInTarget);
 			ApplyTargetPosition();
 
+			if (XamlRoot is not null && _popup.XamlRoot is null)
+			{
+				_popup.XamlRoot = XamlRoot;
+			}
+
 			_popup.IsOpen = true;
 		}