fix(popup): Put Popup in Parent hierarchy of its Child content.

Set Popup as Parent of PopupPanel. This emulates UWP behavior that the Popup is a Parent of one of the parents of the Child content, which is relied upon in some cases (eg WCT DataGrid relies upon this for focus management).

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls_Primitives/Given_Popup.cs

@@ -0,0 +1,89 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls_Primitives.PopupPages;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Media;
+using static Private.Infrastructure.TestServices;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls_Primitives
+{
+	[TestClass]
+	[RunsOnUIThread]
+	public class Given_Popup
+	{
+		[TestMethod]
+		public async Task Check_Can_Reach_Main_Visual_Tree()
+		{
+			var page = new ReachMainTreePage();
+			WindowHelper.WindowContent = page;
+
+			await WindowHelper.WaitForLoaded(page);
+
+			Assert.IsTrue(CanReach(page.DummyTextBlock, page));
+
+			try
+			{
+				page.TargetPopup.IsOpen = true;
+				await WindowHelper.WaitForLoaded(page.PopupButton);
+
+				Assert.IsTrue(CanReach(page.PopupButton, page));
+			}
+			finally
+			{
+				page.TargetPopup.IsOpen = false;
+			}
+		}
+
+#if __ANDROID__
+		[TestMethod]
+		public async Task Check_Can_Reach_Main_Visual_Tree_Alternate_Mode()
+		{
+			var originalConfig = FeatureConfiguration.Popup.UseNativePopup;
+			FeatureConfiguration.Popup.UseNativePopup = !originalConfig;
+			try
+			{
+				await Check_Can_Reach_Main_Visual_Tree();
+			}
+			finally
+			{
+				FeatureConfiguration.Popup.UseNativePopup = originalConfig;
+			}
+		}
+#endif
+
+		private static bool CanReach(DependencyObject startingElement, DependencyObject targetElement)
+		{
+			var currentElement = startingElement;
+			while (currentElement != null)
+			{
+				if (currentElement == targetElement)
+				{
+					return true;
+				}
+
+				// Quoting WCT DataGrid:
+				//		// Walk up the visual tree. Try using the framework element's
+				//		// parent.  We do this because Popups behave differently with respect to the visual tree,
+				//		// and it could have a parent even if the VisualTreeHelper doesn't find it.
+				DependencyObject parent = null;
+				if (currentElement is FrameworkElement fe)
+				{
+					parent = fe.Parent;
+				}
+				if (parent == null)
+				{
+					parent = VisualTreeHelper.GetParent(currentElement);
+				}
+
+				currentElement = parent;
+			}
+
+			// Did not hit targetElement
+			return false;
+		}
+	}
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls_Primitives/PopupPages/ReachMainTreePage.xaml

@@ -0,0 +1,26 @@
+<Page x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls_Primitives.PopupPages.ReachMainTreePage"
+	  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+	  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+	  xmlns:local="using:Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls_Primitives.PopupPages"
+	  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+	  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+	  mc:Ignorable="d"
+	  Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+
+	<Grid>
+		<TextBlock x:Name="DummyTextBlock"
+				   x:FieldModifier="public"
+				   Text="Page with Popup" />
+		<Popup x:Name="TargetPopup"
+			   x:FieldModifier="public">
+			<StackPanel>
+				<TextBlock Text="Popup content" />
+				<Grid>
+					<Button x:Name="PopupButton"
+							x:FieldModifier="public"
+							Content="Button in Popup" />
+				</Grid>
+			</StackPanel>
+		</Popup>
+	</Grid>
+</Page>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls_Primitives/PopupPages/ReachMainTreePage.xaml.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Controls.Primitives;
+using Windows.UI.Xaml.Data;
+using Windows.UI.Xaml.Input;
+using Windows.UI.Xaml.Media;
+using Windows.UI.Xaml.Navigation;
+
+// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=234238
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls_Primitives.PopupPages
+{
+	/// <summary>
+	/// An empty page that can be used on its own or navigated to within a Frame.
+	/// </summary>
+	public sealed partial class ReachMainTreePage : Page
+	{
+		public ReachMainTreePage()
+		{
+			this.InitializeComponent();
+		}
+	}
+}

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

@@ -174,17 +174,18 @@ protected override Size ArrangeOverride(Size finalSize)
 			return finalSize;
 		}
 
-#if __ANDROID__
+		private protected override void OnLoaded()
+		{
+			base.OnLoaded();
+			// Set Parent to the Popup, to obtain the same behavior as UWP that the Popup (and therefore the rest of the main visual tree)
+			// is reachable by scaling the combined Parent/GetVisualParent() hierarchy.
+			this.SetParent(Popup);
+		}
+
 		private protected override void OnUnloaded()
 		{
 			base.OnUnloaded();
-
-			if (FeatureConfiguration.Popup.UseNativePopup)
-			{
-				// Unset parent here, because it doesn't get unset in the usual way because the parent is a native view.
-				this.SetParent(null);
-			}
+			this.SetParent(null);
 		}
-#endif
 	}
 }