fix: Do not destroy the singleton Window for single-window targets (backport #18369)

src/SamplesApp/UITests.Shared/Microsoft_UI_Windowing/SingleWindowClose.xaml

@@ -0,0 +1,16 @@
+<Page
+    x:Class="UITests.Microsoft_UI_Windowing.SingleWindowClose"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:local="using:UITests.Microsoft_UI_Windowing"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    xmlns:controls="using:Microsoft.UI.Xaml.Controls"
+    x:DefaultBindMode="TwoWay"
+    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
+    mc:Ignorable="d">
+
+    <StackPanel Padding="8" Spacing="8">
+        <TextBlock Text="This sample is only relevant to Android and iOS." x:Name="LogTextBlock" />
+    </StackPanel>
+</Page>

src/SamplesApp/UITests.Shared/Microsoft_UI_Windowing/SingleWindowClose.xaml.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.UI.Windowing;
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+using SamplesApp;
+using Uno.Disposables;
+using Uno.UI.Common;
+using Uno.UI.Samples.Controls;
+using Uno.UI.Samples.UITests.Helpers;
+using Windows.Graphics;
+
+#if !WINDOWS_UWP && !WINAPPSDK
+using Uno.UI.Xaml;
+using Uno.UI.Xaml.Controls;
+using WinUICoreServices = Uno.UI.Xaml.Core.CoreServices;
+#endif
+
+namespace UITests.Microsoft_UI_Windowing;
+
+[Sample(
+	"Windowing",
+	IsManualTest = true,
+	Description =
+		"- On Android and iOS, try to back out of this app to the main screen and reopen the app via its icon. " +
+		"You should see the Samples app UI load normally - not a blank screen and the app should also not crash. \r\n" +
+		"- On Android and iOS, try to back out of this app and then activate it by opening an URI like uno-samples-test:something. " +
+		"You should see the Samples app UI load normally - not a blank screen and the app should also not crash. ")]
+public sealed partial class SingleWindowClose : Page
+{
+	public SingleWindowClose()
+	{
+		this.InitializeComponent();
+	}
+}

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

@@ -38,6 +38,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="$(MSBuildThisFileDirectory)Microsoft_UI_Windowing\SingleWindowClose.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
     <Page Include="$(MSBuildThisFileDirectory)Microsoft_UI_Windowing\OverlappedPresenterTests.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
@@ -5473,6 +5477,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)Microsoft_UI_Windowing\AppWindowClosing.xaml.cs">
       <DependentUpon>AppWindowClosing.xaml</DependentUpon>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)Microsoft_UI_Windowing\SingleWindowClose.xaml.cs">
+      <DependentUpon>SingleWindowClose.xaml</DependentUpon>
+    </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Microsoft_UI_Windowing\OverlappedPresenterTests.xaml.cs">
       <DependentUpon>OverlappedPresenterTests.xaml</DependentUpon>
     </Compile>

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

@@ -80,7 +80,7 @@ public virtual void Initialize()
 
 	public virtual void Activate()
 	{
-		if (_isClosed)
+		if (NativeWindowFactory.SupportsMultipleWindows && _isClosed)
 		{
 			throw new InvalidOperationException("Cannot reactivate a closed window.");
 		}
@@ -318,10 +318,18 @@ public bool Close()
 
 			// Window.PrepareToClose();
 
-			// set these to null before marking window as closed as they fail if called after m_bIsClosed is set
-			// because they check if window is closed already
-			Window.SetTitleBar(null);
-			Window.Content = null;
+			if (NativeWindowFactory.SupportsMultipleWindows)
+			{
+				// set these to null before marking window as closed as they fail if called after m_bIsClosed is set
+				// because they check if window is closed already
+				Window.SetTitleBar(null);
+				Window.Content = null;
+			}
+			else
+			{
+				// Just reset the window to not shown state so it can be reactivated
+				_wasShown = false;
+			}
 
 			// _windowChrome.SetDesktopWindow(null);
 
@@ -336,8 +344,11 @@ public bool Close()
 				RaiseWindowVisibilityChangedEvent(false);
 			}
 
-			// Close native window, cleanup, and unregister from hwnd mapping from DXamlCore
-			Shutdown();
+			if (NativeWindowFactory.SupportsMultipleWindows)
+			{
+				// Close native window, cleanup, and unregister from hwnd mapping from DXamlCore
+				Shutdown();
+			}
 
 			return true;
 		}