fix: Popup and Flyout placement within Islands

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

@@ -15,6 +15,7 @@ using Windows.UI.Core;
 using Windows.UI.ViewManagement;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Media;
+using WinUICoreServices = Uno.UI.Xaml.Core.CoreServices;
 
 #if XAMARIN_IOS
 using View = UIKit.UIView;
@@ -328,7 +329,16 @@ namespace Windows.UI.Xaml.Controls.Primitives
 						throw new ArgumentException("Invalid flyout position");
 					}
 
-					var visibleBounds = ApplicationView.GetForCurrentView().VisibleBounds;
+					Rect visibleBounds;
+					if (WinUICoreServices.Instance.InitializationType == Uno.UI.Xaml.Core.InitializationType.IslandsOnly)
+					{
+						var xamlRoot = XamlRoot ?? placementTarget?.XamlRoot;
+						visibleBounds = xamlRoot.Bounds;
+					}
+					else
+					{
+						visibleBounds = ApplicationView.GetForCurrentView().VisibleBounds;
+					}
 					positionValue = new Point(
 						positionValue.X.Clamp(visibleBounds.Left, visibleBounds.Right),
 						positionValue.Y.Clamp(visibleBounds.Top, visibleBounds.Bottom));
@@ -517,6 +527,12 @@ namespace Windows.UI.Xaml.Controls.Primitives
 		{
 			// UNO TODO: UWP also uses values coming from the input pane and app bars, if any.
 			// Make sure of migrate to XamlRoot: https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.xamlroot
+			if (WinUICoreServices.Instance.InitializationType == Uno.UI.Xaml.Core.InitializationType.IslandsOnly)
+			{
+				var xamlRoot = popup.XamlRoot ?? popup.Child?.XamlRoot;
+				return xamlRoot.Bounds;
+			}
+
 			return ApplicationView.GetForCurrentView().VisibleBounds;
 		}
 

src/Uno.UI/UI/Xaml/Controls/Popup/PopupPanel.Placement.cs

@@ -5,6 +5,7 @@ using Windows.UI.ViewManagement;
 using Uno.UI;
 using Uno.Foundation.Logging;
 using System.Linq;
+using WinUICoreServices = Uno.UI.Xaml.Core.CoreServices;
 
 
 #if HAS_UNO_WINUI
@@ -91,7 +92,7 @@ partial class PopupPanel
 
 			if (Flyout?.IsTargetPositionSet ?? false)
 			{
-				rect = Flyout.UpdateTargetPosition(ApplicationView.GetForCurrentView().VisibleBounds, desiredSize, rect);
+				rect = Flyout.UpdateTargetPosition(GetVisibleBounds(), desiredSize, rect);
 			}
 
 			child.Arrange(rect);
@@ -116,7 +117,7 @@ partial class PopupPanel
 	{
 		var anchorRect = GetAnchorRect(popup);
 
-		var visibleBounds = ApplicationView.GetForCurrentView().VisibleBounds;
+		var visibleBounds = GetVisibleBounds();
 
 		// Make sure the desiredSize fits in the panel
 		desiredSize.Width = Math.Min(desiredSize.Width, visibleBounds.Width);
@@ -376,4 +377,8 @@ partial class PopupPanel
 
 		return fits;
 	}
+
+	private Rect GetVisibleBounds() =>
+		WinUICoreServices.Instance.InitializationType == Uno.UI.Xaml.Core.InitializationType.IslandsOnly ?
+			XamlRoot.Bounds : ApplicationView.GetForCurrentView().VisibleBounds;
 }