Add Device Tests for BlazorWebView OnBackPressed handler registration

Tests verify that BlazorWebViewHandler on Android:
1. Registers OnBackPressed lifecycle event handler in ConnectHandler
2. Properly cleans up the handler in DisconnectHandler

Related to: #32767

Author: StephaneDelcroix

src/BlazorWebView/tests/DeviceTests/Elements/BlazorWebViewTests.BackNavigation.cs

@@ -0,0 +1,111 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Components.WebView.Maui;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Maui.LifecycleEvents;
+using Xunit;
+
+namespace Microsoft.Maui.MauiBlazorWebView.DeviceTests.Elements;
+
+public partial class BlazorWebViewTests
+{
+#if ANDROID
+	/// <summary>
+	/// Verifies that BlazorWebViewHandler registers an OnBackPressed lifecycle event handler
+	/// when connected on Android. This handler is essential for proper back navigation within
+	/// the BlazorWebView on Android 13+ with predictive back gestures.
+	/// See: https://github.com/dotnet/maui/issues/32767
+	/// </summary>
+	[Fact]
+	public async Task BlazorWebViewRegistersOnBackPressedHandler()
+	{
+		EnsureHandlerCreated(additionalCreationActions: appBuilder =>
+		{
+			appBuilder.Services.AddMauiBlazorWebView();
+		});
+
+		var bwv = new BlazorWebViewWithCustomFiles
+		{
+			HostPage = "wwwroot/index.html",
+			CustomFiles = new Dictionary<string, string>
+			{
+				{ "index.html", TestStaticFilesContents.DefaultMauiIndexHtmlContent },
+			},
+		};
+		bwv.RootComponents.Add(new RootComponent { ComponentType = typeof(MauiBlazorWebView.DeviceTests.Components.NoOpComponent), Selector = "#app", });
+
+		await InvokeOnMainThreadAsync(async () =>
+		{
+			var bwvHandler = CreateHandler<BlazorWebViewHandler>(bwv);
+			var platformWebView = bwvHandler.PlatformView;
+			await WebViewHelpers.WaitForWebViewReady(platformWebView);
+
+			// Get the lifecycle event service and verify OnBackPressed handler is registered
+			var lifecycleService = MauiContext.Services.GetService<ILifecycleEventService>() as LifecycleEventService;
+			Assert.NotNull(lifecycleService);
+
+			// Verify the OnBackPressed event has been registered
+			Assert.True(lifecycleService.ContainsEvent(nameof(AndroidLifecycle.OnBackPressed)),
+				"BlazorWebViewHandler should register an OnBackPressed lifecycle event handler on Android");
+		});
+	}
+
+	/// <summary>
+	/// Verifies that BlazorWebViewHandler properly cleans up the OnBackPressed lifecycle event handler
+	/// when disconnected. This prevents memory leaks and ensures proper cleanup.
+	/// See: https://github.com/dotnet/maui/issues/32767
+	/// </summary>
+	[Fact]
+	public async Task BlazorWebViewCleansUpOnBackPressedHandlerOnDisconnect()
+	{
+		EnsureHandlerCreated(additionalCreationActions: appBuilder =>
+		{
+			appBuilder.Services.AddMauiBlazorWebView();
+		});
+
+		var bwv = new BlazorWebViewWithCustomFiles
+		{
+			HostPage = "wwwroot/index.html",
+			CustomFiles = new Dictionary<string, string>
+			{
+				{ "index.html", TestStaticFilesContents.DefaultMauiIndexHtmlContent },
+			},
+		};
+		bwv.RootComponents.Add(new RootComponent { ComponentType = typeof(MauiBlazorWebView.DeviceTests.Components.NoOpComponent), Selector = "#app", });
+
+		await InvokeOnMainThreadAsync(async () =>
+		{
+			var bwvHandler = CreateHandler<BlazorWebViewHandler>(bwv);
+			var platformWebView = bwvHandler.PlatformView;
+			await WebViewHelpers.WaitForWebViewReady(platformWebView);
+
+			var lifecycleService = MauiContext.Services.GetService<ILifecycleEventService>() as LifecycleEventService;
+			Assert.NotNull(lifecycleService);
+
+			// Verify handler is registered after connect
+			Assert.True(lifecycleService.ContainsEvent(nameof(AndroidLifecycle.OnBackPressed)),
+				"OnBackPressed handler should be registered after ConnectHandler");
+
+			// Count the handlers before disconnect
+			var handlersBefore = lifecycleService.GetEventDelegates<AndroidLifecycle.OnBackPressed>(nameof(AndroidLifecycle.OnBackPressed));
+			int countBefore = 0;
+			foreach (var _ in handlersBefore)
+				countBefore++;
+
+			// Disconnect the handler by setting the BlazorWebView's Handler to null
+			// This triggers DisconnectHandler internally
+			bwv.Handler = null;
+
+			// Count the handlers after disconnect
+			var handlersAfter = lifecycleService.GetEventDelegates<AndroidLifecycle.OnBackPressed>(nameof(AndroidLifecycle.OnBackPressed));
+			int countAfter = 0;
+			foreach (var _ in handlersAfter)
+				countAfter++;
+
+			// Verify the handler count decreased (cleanup happened)
+			Assert.True(countAfter < countBefore,
+				$"OnBackPressed handler should be removed after DisconnectHandler. Before: {countBefore}, After: {countAfter}");
+		});
+	}
+#endif
+}