feat: Xaml Island root

Author: MartinZikmund

src/Uno.UI/UI/Xaml/Hosting/DesktopWindowXamlSource.cs

@@ -1,15 +1,33 @@
-namespace Windows.UI.Xaml.Hosting;
+using Uno.UI.Xaml.Core;
+using Uno.UI.Xaml.Islands;
+using Windows.UI.Xaml.Controls;
+
+namespace Windows.UI.Xaml.Hosting;
 
 public partial class DesktopWindowXamlSource : global::System.IDisposable
 {
-	public global::Windows.UI.Xaml.UIElement Content { get; set; }
+	private XamlIslandRoot _root;
+
+	public global::Windows.UI.Xaml.UIElement Content
+	{
+		get => _root?.ContentRoot.VisualTree.PublicRootVisual;
+		set
+		{
+			if (_root is null)
+			{
+				_root = new XamlIslandRoot(CoreServices.Instance);
+			}
+			_root.SetPublicRootVisual(value);
+		}
+	}	
 
 	public bool HasFocus => false;
 
 	public DesktopWindowXamlSource()
 	{
-		global::Windows.Foundation.Metadata.ApiInformation.TryRaiseNotImplemented("Windows.UI.Xaml.Hosting.DesktopWindowXamlSource", "DesktopWindowXamlSource.DesktopWindowXamlSource()");
+		
 	}
+
 	public global::Windows.UI.Xaml.Hosting.XamlSourceFocusNavigationResult NavigateFocus(global::Windows.UI.Xaml.Hosting.XamlSourceFocusNavigationRequest request)
 	{
 		return null;

src/Uno.UI/UI/Xaml/Internal/CoreServices.cs

@@ -62,6 +62,7 @@ internal void PutVisualRoot(DependencyObject? dependencyObject)
 
 			// Set the root visual from the parser result. If we're passed null it means
 			// we're supposed to just clear the tree.
+			// TODO: This is not currently happening, adjust when porting next time
 			if (dependencyObject != null)
 			{
 				var root = dependencyObject as UIElement;

src/Uno.UI/UI/Xaml/Internal/Islands/XamlIslandRoot.cs

@@ -0,0 +1,30 @@
+using Uno.UI.Xaml.Core;
+using Windows.UI;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+
+namespace Uno.UI.Xaml.Islands;
+
+internal class XamlIslandRoot : Panel
+{
+	private readonly ContentRoot _contentRoot;
+
+	internal XamlIslandRoot(CoreServices coreServices)
+	{
+		_contentRoot = coreServices.ContentRootCoordinator.CreateContentRoot(ContentRootType.XamlIsland, Colors.Transparent, this);
+
+		//Uno specific - flag as VisualTreeRoot for interop with existing logic
+		IsVisualTreeRoot = true;
+	}
+
+	internal ContentRoot ContentRoot => _contentRoot;
+
+	internal void SetPublicRootVisual(UIElement uiElement)
+	{
+		_contentRoot.VisualTree.SetPublicRootVisual(uiElement, null, null);
+
+		UIElement.LoadingRootElement(uiElement);
+
+		UIElement.RootElementLoaded(uiElement);
+	}
+}