feat(flyout): Implement FlyoutBase.OverlayInputPassThroughElement

src/SamplesApp/UITests.Shared/UITests.Shared.projitems

@@ -1834,6 +1834,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\Flyout\Flyout_OverlayInputPassThroughElement.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
     <Page Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\FlipView\FlipView_Images.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
@@ -6099,6 +6103,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\GridView\GridViewMultipleSelectionMode.xaml.cs">
       <DependentUpon>GridViewMultipleSelectionMode.xaml</DependentUpon>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\Flyout\Flyout_OverlayInputPassThroughElement.xaml.cs">
+      <DependentUpon>Flyout_OverlayInputPassThroughElement.xaml</DependentUpon>
+    </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\ImageTests\ImageSourceWriteableBitmapInvalidate_Stretch.xaml.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\IconSourceTests\IconSourceElementTests.xaml.cs">
       <DependentUpon>IconSourceElementTests.xaml</DependentUpon>

src/Uno.UI/Generated/3.0.0.0/Microsoft.UI.Xaml.Controls.Primitives/FlyoutBase.cs

@@ -38,7 +38,7 @@ public bool ShouldConstrainToRootBounds
 		}
 #endif
 		// Skipping already declared property Placement
-#if __ANDROID__ || __IOS__ || IS_UNIT_TESTS || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
+#if __ANDROID__ || __IOS__ || IS_UNIT_TESTS || __WASM__ || false || __NETSTD_REFERENCE__ || __MACOS__
 		[global::Uno.NotImplemented("__ANDROID__", "__IOS__", "IS_UNIT_TESTS", "__WASM__", "__SKIA__", "__NETSTD_REFERENCE__", "__MACOS__")]
 		public global::Microsoft.UI.Xaml.DependencyObject OverlayInputPassThroughElement
 		{
@@ -148,7 +148,7 @@ public bool IsConstrainedToRootBounds
 #endif
 		// Skipping already declared property IsOpenProperty
 		// Skipping already declared property LightDismissOverlayModeProperty
-#if __ANDROID__ || __IOS__ || IS_UNIT_TESTS || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
+#if __ANDROID__ || __IOS__ || IS_UNIT_TESTS || __WASM__ || false || __NETSTD_REFERENCE__ || __MACOS__
 		[global::Uno.NotImplemented("__ANDROID__", "__IOS__", "IS_UNIT_TESTS", "__WASM__", "__SKIA__", "__NETSTD_REFERENCE__", "__MACOS__")]
 		public static global::Microsoft.UI.Xaml.DependencyProperty OverlayInputPassThroughElementProperty { get; } =
 		Microsoft.UI.Xaml.DependencyProperty.Register(

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

@@ -244,6 +244,15 @@ internal Brush LightDismissOverlayBackground
 
 		internal static DependencyProperty LightDismissOverlayBackgroundProperty { get; } =
 			DependencyProperty.Register("LightDismissOverlayBackground", typeof(Brush), typeof(FlyoutBase), new FrameworkPropertyMetadata(null));
+
+		internal static DependencyProperty OverlayInputPassThroughElementProperty { get; } = 
+			DependencyProperty.Register("OverlayInputPassThroughElement", typeof(DependencyObject), typeof(FlyoutBase), new FrameworkPropertyMetadata(null));
+
+		public DependencyObject OverlayInputPassThroughElement
+		{
+			get { return (DependencyObject)GetValue(OverlayInputPassThroughElementProperty); }
+			set { SetValue(OverlayInputPassThroughElementProperty, value); }
+		}
 
 		/// <summary>
 		/// Gets or sets whether a disabled control can receive focus.

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

@@ -1,11 +1,14 @@
 #if !__UWP__
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Text;
 using Uno.UI;
 using Windows.Foundation;
+using Windows.UI.Core;
 using Windows.UI.ViewManagement;
 using Microsoft.UI.Xaml.Controls.Primitives;
+using Microsoft.UI.Xaml.Input;
 using Microsoft.UI.Xaml.Media;
 
 namespace Microsoft.UI.Xaml.Controls
@@ -35,6 +38,30 @@ public FlyoutBasePopupPanel(FlyoutBase flyout) : base(flyout._popup)
 		internal override FlyoutBase Flyout => _flyout;
 
 		protected override int PopupPlacementTargetMargin => 5;
+
+		protected override void OnPointerPressed(object sender, PointerRoutedEventArgs args)
+		{
+			base.OnPointerPressed(sender, args);
+
+			// Make sure we are the original source.  We do not want to handle PointerPressed on the Popup itself.
+			if (args.OriginalSource == this)
+			{
+				if (Flyout.OverlayInputPassThroughElement is UIElement passThroughElement)
+				{
+					var (elementToBeHit, _) = VisualTreeHelper.SearchDownForTopMostElementAt(
+						args.GetCurrentPoint(null).Position,
+						passThroughElement.XamlRoot.VisualTree.RootElement,
+						VisualTreeHelper.DefaultGetTestability,
+						childrenFilter: elements => elements.Where(e => e != this));
+
+					var eventArgs = new PointerRoutedEventArgs(
+						new PointerEventArgs(args.GetCurrentPoint(null), args.KeyModifiers),
+						args.OriginalSource as UIElement);
+
+					elementToBeHit.OnPointerDown(eventArgs);
+				}
+			}
+		}
 	}
 }
 #endif

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

@@ -243,7 +243,7 @@ private protected override void OnUnloaded()
 
 	// TODO: pointer handling should really go on PopupRoot. For now it's easier to put here because PopupRoot doesn't track open popups, and also we
 	// need to support native popups on Android that don't use PopupRoot.
-	private void OnPointerPressed(object sender, PointerRoutedEventArgs args)
+	protected virtual void OnPointerPressed(object sender, PointerRoutedEventArgs args)
 	{
 		// Make sure we are the original source.  We do not want to handle PointerPressed on the Popup itself.
 		if (args.OriginalSource == this && Popup is { } popup)

src/Uno.UI/UI/Xaml/Media/VisualTreeHelper.cs

@@ -448,7 +448,7 @@ internal static (UIElement? element, Branch? stale) HitTest(
 		/// On skia: The absolute position relative to the window origin.
 		/// Everywhere else: The position relative to the parent (i.e. the position in parent coordinates).
 		/// </param>
-		private static (UIElement? element, Branch? stale) SearchDownForTopMostElementAt(
+		internal static (UIElement? element, Branch? stale) SearchDownForTopMostElementAt(
 			Point position,
 			UIElement element,
 			GetHitTestability getVisibility,