Added a UI test (dotnet#21948)

src/Controls/tests/TestCases.HostApp/Issues/Issue21948.xaml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Maui.Controls.Sample.Issues.Issue21948">
+    <Button Text="Open a new window"
+            AutomationId="button"
+            Clicked="OpenNewWindowClicked" />
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue21948.xaml.cs

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+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+using Microsoft.Maui.Platform;
+using Microsoft.Maui;
+using System.Linq;
+using System;
+using System.Threading.Tasks;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+#if IOS || MACCATALYST
+using UIKit;
+using CoreGraphics;
+#endif
+
+namespace Maui.Controls.Sample.Issues
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.ManualTest, 21948, "Crash upon resuming the app 'window was already activated'", PlatformAffected.iOS)]
+	public partial class Issue21948 : ContentPage
+	{
+		public Issue21948()
+		{
+			InitializeComponent();
+		}
+
+		public void OpenNewWindowClicked(object obj, EventArgs e)
+		{
+#if IOS || MACCATALYST
+			OpenNewWindow();
+#endif
+		}
+
+#if IOS || MACCATALYST
+		async void OpenNewWindow()
+        {
+            var uIWindow = new UIWindow();
+            var keyWindow = (this.Window.Handler.PlatformView as UIWindow);
+            if (keyWindow?.WindowLevel == UIWindowLevel.Normal)
+                keyWindow.WindowLevel = -1;
+
+            var page = new ContentPage();
+            this.AddLogicalChild(page);
+            var handler = page.ToHandler(this.Handler.MauiContext);
+
+            uIWindow.RootViewController = new UIViewController();
+            uIWindow.WindowLevel = UIWindowLevel.Normal;
+            uIWindow.MakeKeyAndVisible();
+
+            // Simulate backgrounding the app
+            nint taskId = UIApplication.BackgroundTaskInvalid;
+            taskId = UIApplication.SharedApplication.BeginBackgroundTask(() =>
+            {
+                UIApplication.SharedApplication.EndBackgroundTask(taskId);
+            });
+
+			// Simulate background time
+            await Task.Delay(2000); 
+            UIApplication.SharedApplication.EndBackgroundTask(taskId);
+
+			var rvc = uIWindow.RootViewController;
+
+            if (rvc != null)
+            {
+                await rvc.DismissViewControllerAsync(false);
+                rvc.Dispose();
+            }
+
+            // Simulate bringing the app back to the foreground
+            await Task.Delay(1000);
+
+            uIWindow.RootViewController = null;
+            uIWindow.Hidden = true;
+            keyWindow.WindowLevel = UIWindowLevel.Normal;
+            this.RemoveLogicalChild(page);
+		}
+#endif
+	}
+
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue23360.cs

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+#if IOS || MACCATALYST
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue21948 : _IssuesUITest
+	{
+		public Issue21948(TestDevice device) : base(device) { }
+
+		public override string Issue => "Crash upon resuming the app 'window was already activated'";
+
+		[Test]
+		public void OpenAlertWithModals()
+		{
+			App.WaitForElement("button");
+			App.Click("button");
+
+			// The test passes if no exception is thrown
+			App.WaitForElement("button");
+		}
+	}
+}
+#endif