fix: Avoid assigning secondary window in Uno Islands Main window ID

src/Uno.UI.XamlHost.Skia.Wpf/UnoXamlHostBase.cs

@@ -13,6 +13,7 @@
 using Uno.UI.Xaml.Controls;
 using Microsoft.UI.Content;
 using Uno.UI.Xaml.Core;
+using Microsoft.UI.Windowing;
 
 namespace Uno.UI.XamlHost.Skia.Wpf
 {
@@ -76,6 +77,8 @@ static UnoXamlHostBase()
 							application = (WUX.Application)Activator.CreateInstance(type);
 						});
 
+						AppWindow.SkipMainWindowId();
+
 						return (WUX.Markup.IXamlMetadataProvider)application;
 					}
 

src/Uno.UI/UI/Xaml/Window/Window.cs

@@ -52,7 +52,7 @@ partial class Window
 
 	internal Window(WindowType windowType)
 	{
-		if (_current is null)
+		if (_current is null && CoreApplication.IsFullFledgedApp)
 		{
 			windowType = WindowType.CoreWindow;
 		}

src/Uno.UWP/Microsoft/UI/Windowing/AppWindow.cs

@@ -7,6 +7,8 @@
 using Windows.Graphics;
 using Windows.UI.ViewManagement;
 using MUXWindowId = Microsoft.UI.WindowId;
+using Windows.ApplicationModel.Core;
+
 
 #if HAS_UNO_WINUI
 using Microsoft.UI.Dispatching;
@@ -103,6 +105,17 @@ public string Title
 		}
 	}
 
+	internal static void SkipMainWindowId()
+	{
+		// In case of Uno Islands we currently have no "main window",
+		// so we must avoid assigning the first created secondary window
+		// Id = 1, otherwise it would be considered as the main window.
+		if (!CoreApplication.IsFullFledgedApp && _windowIdIterator == 0)
+		{
+			_windowIdIterator++;
+		}
+	}
+
 	internal void SetNativeWindow(INativeAppWindow nativeAppWindow)
 	{
 		if (nativeAppWindow is null)