fix(popup): not working after alt-tabbing

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

@@ -1589,6 +1589,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\MenuFlyoutTests\MenuFlyout_DroidAltTab.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
     <Page Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\MenuFlyoutTests\MenuFlyout_IosNative.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
@@ -5457,6 +5461,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\MenuFlyoutTests\MenuFlyoutItem_Hierarchy.xaml.cs">
       <DependentUpon>MenuFlyoutItem_Hierarchy.xaml</DependentUpon>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\MenuFlyoutTests\MenuFlyout_DroidAltTab.xaml.cs">
+      <DependentUpon>MenuFlyout_DroidAltTab.xaml</DependentUpon>
+    </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Windows_UI_Xaml_Controls\MenuFlyoutTests\MenuFlyout_IosNative.xaml.cs">
       <DependentUpon>MenuFlyout_IosNative.xaml</DependentUpon>
     </Compile>

src/SamplesApp/UITests.Shared/Windows_UI_Xaml_Controls/MenuFlyoutTests/MenuFlyout_DroidAltTab.xaml

@@ -0,0 +1,26 @@
+<UserControl x:Class="UITests.Windows_UI_Xaml_Controls.MenuFlyoutTests.MenuFlyout_DroidAltTab"
+			 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+			 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+			 xmlns:local="using:UITests.Windows_UI_Xaml_Controls.MenuFlyoutTests"
+			 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+			 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+			 xmlns:android="http://uno.ui/android"
+			 mc:Ignorable="d android"
+			 d:DesignHeight="300"
+			 d:DesignWidth="400">
+	<Grid>
+		<CommandBar Content="asd" Style="{StaticResource NativeDefaultCommandBar}">
+			<CommandBar.PrimaryCommands>
+				<AppBarButton Content="Here">
+					<AppBarButton.Flyout>
+						<MenuFlyout android:UseNativePopup="True">
+							<MenuFlyoutItem Text="asd1" />
+							<MenuFlyoutItem Text="asd2" />
+							<MenuFlyoutItem Text="asd3" />
+						</MenuFlyout>
+					</AppBarButton.Flyout>
+				</AppBarButton>
+			</CommandBar.PrimaryCommands>
+		</CommandBar>
+	</Grid>
+</UserControl>

src/SamplesApp/UITests.Shared/Windows_UI_Xaml_Controls/MenuFlyoutTests/MenuFlyout_DroidAltTab.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 User Control item template is documented at https://go.microsoft.com/fwlink/?LinkId=234236
+
+namespace UITests.Windows_UI_Xaml_Controls.MenuFlyoutTests
+{
+	[Uno.UI.Samples.Controls.SampleControlInfo("MenuFlyout", nameof(MenuFlyout_DroidAltTab), description: Description, IsManualTest = true, IgnoreInSnapshotTests = true)]
+	public sealed partial class MenuFlyout_DroidAltTab : UserControl
+	{
+		private const string Description = "[ManualTest]: tap 'Here' to open popup, switch app/go to home and come back, tap `Here` and popup should still work.";
+
+		public MenuFlyout_DroidAltTab()
+		{
+			this.InitializeComponent();
+		}
+	}
+}

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

@@ -9,12 +9,16 @@
 using Windows.UI.Xaml.Controls.Primitives;
 using Windows.UI.Xaml.Media;
 using Uno.UI;
+using Android.Content;
+using Uno.Disposables;
 
 namespace Windows.UI.Xaml.Controls.Primitives
 {
 	public partial class Popup
 	{
+		private readonly SerialDisposable _disposable = new SerialDisposable();
 		private PopupWindow _popupWindow;
+		private WeakReference<Context> _popupContextReference;
 
 		internal FlyoutPlacementMode Placement { get; set; }
 
@@ -26,7 +30,8 @@ internal Popup(bool useNativePopup) : this()
 
 		partial void InitializeNativePartial()
 		{
-			_popupWindow = new PopupWindow(this, WindowManagerLayoutParams.MatchParent, WindowManagerLayoutParams.MatchParent, true);
+			_popupWindow = new PopupWindow(ApplicationActivity.Instance);
+			_popupContextReference = new WeakReference<Context>(ApplicationActivity.Instance);
 
 			_popupWindow.Width = WindowManagerLayoutParams.MatchParent;
 			_popupWindow.Height = WindowManagerLayoutParams.MatchParent;
@@ -36,6 +41,19 @@ internal Popup(bool useNativePopup) : this()
 			OnIsLightDismissEnabledChanged(false, true);
 
 			_popupWindow.DismissEvent += OnPopupDismissed;
+			_disposable.Disposable = Disposable.Create(() => _popupWindow.DismissEvent -= OnPopupDismissed);
+		}
+
+		private void ReinitializePopupWindowIfContextChanged()
+		{
+			if (_popupWindow != null &&
+				_popupContextReference?.TryGetTarget(out var popupContextReference) == true &&
+				popupContextReference != ApplicationActivity.Instance)
+			{
+				_disposable.Dispose();
+				InitializeNativePartial();
+				OnPopupPanelChangedPartialNative(PopupPanel, PopupPanel);
+			}
 		}
 
 		partial void OnPopupPanelChangedPartialNative(PopupPanel previousPanel, PopupPanel newPanel)
@@ -63,6 +81,8 @@ private void OnPopupDismissed(object sender, EventArgs e)
 
 		partial void OnIsOpenChangedNative(bool oldIsOpen, bool newIsOpen)
 		{
+			ReinitializePopupWindowIfContextChanged();
+
 			if (newIsOpen)
 			{
 				PopupPanel.Visibility = Visibility.Visible;