Fix transparent default modal background

src/Controls/src/Core/Platform/ModalNavigationManager/ModalNavigationManager.Android.cs

@@ -16,6 +16,7 @@
 using AndroidX.AppCompat.App;
 using Microsoft.Maui.LifecycleEvents;
 using AndroidX.Activity;
+using System.ComponentModel;
 
 namespace Microsoft.Maui.Controls.Platform
 {
@@ -241,6 +242,8 @@ internal class ModalFragment : DialogFragment
 			public ModalFragment(IMauiContext mauiContext, Page modal)
 			{
 				_modal = modal;
+				_modal.PropertyChanged += OnModalPagePropertyChanged;
+				_modal.HandlerChanged += OnPageHandlerChanged;
 				_mauiWindowContext = mauiContext;
 			}
 
@@ -256,6 +259,53 @@ public ModalFragment(IMauiContext mauiContext, Page modal)
 				return dialog;
 			}
 
+			void OnPageHandlerChanged(object? sender, EventArgs e)
+			{
+				if (sender is Page page)
+				{
+					page.HandlerChanged -= OnPageHandlerChanged;
+				}
+
+				UpdateBackgroundColor();
+			}
+
+			void OnModalPagePropertyChanged(object? sender, PropertyChangedEventArgs e)
+			{
+				if (_modal is null)
+				{
+					if (sender is Page page)
+					{
+						page.PropertyChanged -= OnModalPagePropertyChanged;
+						page.HandlerChanged -= OnPageHandlerChanged;
+					}
+
+					return;
+				}
+
+
+				if (e.IsOneOf(Page.BackgroundColorProperty, Page.BackgroundProperty))
+				{
+					UpdateBackgroundColor();
+				}
+			}
+
+			void UpdateBackgroundColor()
+			{
+				if (_modal is not IView view || view.Handler is not IPlatformViewHandler platformViewHandler)
+				{
+					return;
+				}
+
+				var pageView = platformViewHandler.PlatformView;
+
+				if (pageView is null)
+					return;
+
+				var modalBkgndColor = view.Background;
+				if (modalBkgndColor is null)
+					pageView.SetWindowBackground();
+			}
+
 			public override AView OnCreateView(LayoutInflater inflater, ViewGroup? container, Bundle? savedInstanceState)
 			{
 				var modalContext = _mauiWindowContext
@@ -264,6 +314,8 @@ public override AView OnCreateView(LayoutInflater inflater, ViewGroup? container
 				_navigationRootManager = modalContext.GetNavigationRootManager();
 				_navigationRootManager.Connect(_modal, modalContext);
 
+				UpdateBackgroundColor();
+
 				return _navigationRootManager?.RootView ??
 					throw new InvalidOperationException("Root view not initialized");
 			}
@@ -309,6 +361,9 @@ void OnAnimationEnded(object? sender, AAnimation.AnimationEndEventArgs e)
 
 			public override void OnDismiss(IDialogInterface dialog)
 			{
+				_modal.PropertyChanged -= OnModalPagePropertyChanged;
+				_modal.HandlerChanged -= OnPageHandlerChanged;
+
 				if (_modal.Toolbar?.Handler is not null)
 				{
 					_modal.Toolbar.Handler = null;

src/Controls/tests/TestCases.HostApp/Issues/Issue23973.cs

@@ -0,0 +1,75 @@
+namespace Maui.Controls.Sample.Issues;
+
+[XamlCompilation(XamlCompilationOptions.Compile)]
+[Issue(IssueTracker.Github, "23973", "Default Modal Page Is Not Transparent", PlatformAffected.All)]
+public partial class Issue23973 : ContentPage
+{
+	public Issue23973()
+	{
+        Content = new VerticalStackLayout()
+        {
+            new Label(){
+                Text = "Label 1"
+            },
+            new Button()
+            {
+                Text = "Click to show opaque default modal page",
+                Command = new Command(async () => await Navigation.PushModalAsync(new ModalPage())),
+				LineBreakMode = LineBreakMode.WordWrap,
+                AutomationId = "PushModal"
+            },
+            new Button()
+            {
+                Text = "Click to show transparent modal page",
+                Command = new Command(async () => await Navigation.PushModalAsync(new TransparentModalPage())),
+				LineBreakMode = LineBreakMode.WordWrap,
+                AutomationId = "PushTransparentModal"
+            },
+            new Label(){
+                Text = "Label 2"
+            },
+            new Label(){
+                Text = "Label 3"
+            },
+        };
+	}
+
+    public class TransparentModalPage : ContentPage
+	{
+		public TransparentModalPage()
+		{
+			Title = "Transparent Modal Page";
+            BackgroundColor = Colors.Transparent;
+
+			Content = new VerticalStackLayout()
+			{
+				new Button() 
+				{
+					AutomationId = "PopModal",
+					Text = "If you do not see through to the underlying page, this test has failed",
+					LineBreakMode = LineBreakMode.WordWrap,
+					Command = new Command(async () => await Navigation.PopModalAsync())
+				}
+			};
+		}
+	}
+
+	public class ModalPage : ContentPage
+	{
+		public ModalPage()
+		{
+			Title = "Opaque Modal Page";
+
+			Content = new VerticalStackLayout()
+			{
+				new Button() 
+				{
+					AutomationId = "PopModal",
+					Text = "If you see through to the underlying page, this test has failed",
+					LineBreakMode = LineBreakMode.WordWrap,
+					Command = new Command(async () => await Navigation.PopModalAsync())
+				}
+			};
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue23973.cs

@@ -0,0 +1,34 @@
+#if !MACCATALYST // MACCATALYST doesn't support VerifyScreenshot tests
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue23973(TestDevice device) : _IssuesUITest(device)
+	{
+		protected override bool ResetAfterEachTest => true;
+		public override string Issue => "Default Modal Page Is Not Transparent"; 
+
+		[Test]
+		[Category(UITestCategories.Navigation)]
+		public void VerifyOpaqueModalDefault()
+		{
+			App.WaitForElement("PushModal");
+			App.Tap("PushModal");
+			VerifyScreenshot();
+		}
+
+#if !WINDOWS
+		[Test]
+		[Category(UITestCategories.Navigation)]
+		public void VerifyTransparentModalShowsPageBeneathModal()
+		{
+			App.WaitForElement("PushTransparentModal");
+			App.Tap("PushTransparentModal");
+			VerifyScreenshot();
+		}
+#endif
+	}
+}
+#endif