[Issue-Resolver] Explicit fallback for BackButtonBehavior lookup

src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ShellToolbarTracker.cs

@@ -159,7 +159,7 @@ protected SearchHandler SearchHandler
 
 		void AView.IOnClickListener.OnClick(AView v)
 		{
-			var backButtonHandler = Shell.GetBackButtonBehavior(Page);
+			var backButtonHandler = Shell.GetEffectiveBackButtonBehavior(Page);
 			var isEnabled = backButtonHandler.GetPropertyIfSet(BackButtonBehavior.IsEnabledProperty, true);
 
 			if (isEnabled)
@@ -255,7 +255,7 @@ protected virtual void OnPageChanged(Page oldPage, Page newPage)
 			if (newPage != null)
 			{
 				newPage.PropertyChanged += OnPagePropertyChanged;
-				_backButtonBehavior = Shell.GetBackButtonBehavior(newPage);
+				_backButtonBehavior = Shell.GetEffectiveBackButtonBehavior(newPage);
 
 				if (_backButtonBehavior != null)
 					_backButtonBehavior.PropertyChanged += OnBackButtonBehaviorChanged;
@@ -309,7 +309,7 @@ protected virtual void OnPagePropertyChanged(object sender, PropertyChangedEvent
 				UpdateNavBarHasShadow(Page);
 			else if (e.PropertyName == Shell.BackButtonBehaviorProperty.PropertyName)
 			{
-				var backButtonHandler = Shell.GetBackButtonBehavior(Page);
+				var backButtonHandler = Shell.GetEffectiveBackButtonBehavior(Page);
 
 				if (_backButtonBehavior != null)
 					_backButtonBehavior.PropertyChanged -= OnBackButtonBehaviorChanged;
@@ -407,7 +407,7 @@ protected virtual async void UpdateLeftBarButtonItem(Context context, AToolbar t
 				drawerLayout.AddDrawerListener(_drawerToggle);
 			}
 
-			var backButtonHandler = Shell.GetBackButtonBehavior(page);
+			var backButtonHandler = Shell.GetEffectiveBackButtonBehavior(page);
 			var text = backButtonHandler.GetPropertyIfSet(BackButtonBehavior.TextOverrideProperty, String.Empty);
 			var command = backButtonHandler.GetPropertyIfSet<ICommand>(BackButtonBehavior.CommandProperty, null);
 			var backButtonVisibleFromBehavior = backButtonHandler.GetPropertyIfSet(BackButtonBehavior.IsVisibleProperty, true);
@@ -540,7 +540,7 @@ protected virtual Task UpdateDrawerArrow(Context context, AToolbar toolbar, Draw
 
 		protected virtual void UpdateToolbarIconAccessibilityText(AToolbar toolbar, Shell shell)
 		{
-			var backButtonHandler = Shell.GetBackButtonBehavior(Page);
+			var backButtonHandler = Shell.GetEffectiveBackButtonBehavior(Page);
 			var image = GetFlyoutIcon(backButtonHandler, Page);
 			var text = backButtonHandler.GetPropertyIfSet(BackButtonBehavior.TextOverrideProperty, String.Empty);
 			var automationId = image?.AutomationId ?? text;

src/Controls/src/Core/Compatibility/Handlers/Shell/iOS/ShellPageRendererTracker.cs

@@ -133,7 +133,7 @@ protected virtual void OnPagePropertyChanged(object sender, PropertyChangedEvent
 #nullable restore
 			if (e.PropertyName == Shell.BackButtonBehaviorProperty.PropertyName)
 			{
-				SetBackButtonBehavior(Shell.GetBackButtonBehavior(Page));
+				SetBackButtonBehavior(Shell.GetEffectiveBackButtonBehavior(Page));
 			}
 			else if (e.PropertyName == Shell.SearchHandlerProperty.PropertyName)
 			{
@@ -217,7 +217,7 @@ void UpdateShellToMyPage()
 				return;
 			}
 
-			SetBackButtonBehavior(Shell.GetBackButtonBehavior(Page));
+			SetBackButtonBehavior(Shell.GetEffectiveBackButtonBehavior(Page));
 			SearchHandler = Shell.GetSearchHandler(Page);
 			UpdateTitleView();
 			UpdateTitle();

src/Controls/src/Core/Compatibility/Handlers/Shell/iOS/ShellSectionRenderer.cs

@@ -129,7 +129,7 @@ internal bool SendPop()
 			{
 				if (tracker.Value.ViewController == TopViewController)
 				{
-					var behavior = Shell.GetBackButtonBehavior(tracker.Value.Page);
+					var behavior = Shell.GetEffectiveBackButtonBehavior(tracker.Value.Page);
 					var command = behavior.GetPropertyIfSet<ICommand>(BackButtonBehavior.CommandProperty, null);
 					var commandParameter = behavior.GetPropertyIfSet<object>(BackButtonBehavior.CommandParameterProperty, null);
 

src/Controls/src/Core/Internals/PropertyPropagationExtensions.cs

@@ -40,9 +40,6 @@ internal static void PropagatePropertyChanged(string propertyName, Element eleme
 
 			if (propertyName == null || propertyName == Shell.NavBarVisibilityAnimationEnabledProperty.PropertyName)
 				BaseShellItem.PropagateFromParent(Shell.NavBarVisibilityAnimationEnabledProperty, element);
-
-			if (propertyName == null || propertyName == Shell.BackButtonBehaviorProperty.PropertyName)
-				BaseShellItem.PropagateFromParent(Shell.BackButtonBehaviorProperty, element);
 
 			foreach (var child in children.ToArray())
 			{

src/Controls/src/Core/Shell/Shell.cs

@@ -202,6 +202,34 @@ static void OnFlyoutItemIsVisibleChanged(BindableObject bindable, object oldValu
 		/// <returns>The back button behavior for the object.</returns>
 		public static BackButtonBehavior GetBackButtonBehavior(BindableObject obj) => (BackButtonBehavior)obj.GetValue(BackButtonBehaviorProperty);
 
+		/// <summary>
+		/// Gets the BackButtonBehavior for the given page, with fallback to Shell if not set on the page.
+		/// </summary>
+		internal static BackButtonBehavior GetEffectiveBackButtonBehavior(BindableObject page)
+		{
+			if (page == null)
+				return null;
+
+			// First check if the page has its own BackButtonBehavior
+			var behavior = GetBackButtonBehavior(page);
+			if (behavior != null)
+				return behavior;
+
+			// Fallback: check if the Shell itself has a BackButtonBehavior
+			if (page is Element element)
+			{
+				var shell = element.FindParentOfType<Shell>();
+				if (shell != null)
+				{
+					behavior = GetBackButtonBehavior(shell);
+					if (behavior != null)
+						return behavior;
+				}
+			}
+
+			return null;
+		}
+
 		/// <summary>
 		/// Sets the back button behavior when the given <paramref name="obj"/> is presented.
 		/// </summary>
@@ -1556,7 +1584,7 @@ internal void SendStructureChanged()
 		protected override bool OnBackButtonPressed()
 		{
 #if WINDOWS || !PLATFORM
-			var backButtonBehavior = GetBackButtonBehavior(GetVisiblePage());
+			var backButtonBehavior = GetEffectiveBackButtonBehavior(GetVisiblePage());
 			if (backButtonBehavior != null)
 			{
 				var command = backButtonBehavior.GetPropertyIfSet<ICommand>(BackButtonBehavior.CommandProperty, null);

src/Controls/src/Core/ShellToolbar.cs

@@ -131,7 +131,7 @@ internal void ApplyChanges()
 
 		void UpdateBackbuttonBehavior()
 		{
-			var bbb = Shell.GetBackButtonBehavior(_currentPage);
+			var bbb = Shell.GetEffectiveBackButtonBehavior(_currentPage);
 
 			if (bbb == _backButtonBehavior)
 				return;

src/Controls/tests/TestCases.HostApp/Issues/Issue33688.cs

@@ -0,0 +1,174 @@
+using System.Collections.ObjectModel;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+using System.Windows.Input;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 33688, "BackButtonBehavior is no longer triggered once a ContentPage contains a CollectionView and the ItemsSource has been changed", PlatformAffected.iOS | PlatformAffected.Android)]
+public class Issue33688 : Shell
+{
+	public Issue33688()
+	{
+		var mainContent = new ShellContent
+		{
+			ContentTemplate = new DataTemplate(() => new Issue33688MainPage()),
+			Title = "Main",
+			Route = "main"
+		};
+
+		Items.Add(mainContent);
+		Routing.RegisterRoute("Issue33688_second", typeof(Issue33688SecondPage));
+	}
+}
+
+file class Issue33688MainPage : ContentPage
+{
+	public Issue33688MainPage()
+	{
+		Padding = 24;
+
+		var resultLabel = new Label
+		{
+			Text = "Waiting for back button...",
+			AutomationId = "ResultLabel"
+		};
+
+		// Store reference so ViewModel can update it
+		Issue33688ViewModel.SetResultLabelRef(resultLabel);
+
+		Content = new VerticalStackLayout
+		{
+			Spacing = 10,
+			Children =
+			{
+				new Label
+				{
+					Text = "Tap the button to navigate to a page with a CollectionView. Then press back - the BackButtonBehavior command should fire and update the label below.",
+					AutomationId = "InstructionLabel"
+				},
+				new Button
+				{
+					Text = "Navigate to other Page",
+					AutomationId = "NavigateButton",
+					Command = new Command(() => Shell.Current.GoToAsync("Issue33688_second"))
+				},
+				resultLabel
+			}
+		};
+	}
+}
+
+file class Issue33688SecondPage : ContentPage
+{
+	public Issue33688SecondPage()
+	{
+		// Use a ViewModel pattern with binding - this is the scenario from the issue
+		var viewModel = new Issue33688ViewModel();
+		BindingContext = viewModel;
+
+		// BackButtonBehavior with BOUND Command (key to reproduction)
+		var backButtonBehavior = new BackButtonBehavior();
+		backButtonBehavior.SetBinding(BackButtonBehavior.CommandProperty, nameof(Issue33688ViewModel.SaveAndNavigateBackCommand));
+		Shell.SetBackButtonBehavior(this, backButtonBehavior);
+
+		var collectionView = new CollectionView
+		{
+			AutomationId = "TestCollectionView",
+			ItemTemplate = new DataTemplate(() =>
+			{
+				var label = new Label();
+				label.SetBinding(Label.TextProperty, "Name");
+				return label;
+			})
+		};
+		collectionView.SetBinding(CollectionView.ItemsSourceProperty, nameof(Issue33688ViewModel.Items));
+
+		var filterButton = new Button
+		{
+			Text = "Load Items (triggers bug)",
+			AutomationId = "FilterButton"
+		};
+		filterButton.SetBinding(Button.CommandProperty, nameof(Issue33688ViewModel.FilterCommand));
+
+		var statusLabel = new Label
+		{
+			Text = "Tap 'Load Items' then press back button.",
+			AutomationId = "StatusLabel"
+		};
+
+		Content = new VerticalStackLayout
+		{
+			Padding = 24,
+			Spacing = 10,
+			Children =
+			{
+				statusLabel,
+				filterButton,
+				collectionView
+			}
+		};
+	}
+}
+
+file class Issue33688ViewModel : INotifyPropertyChanged
+{
+	static Label _resultLabelRef = null!;
+
+	public static void SetResultLabelRef(Label label) => _resultLabelRef = label;
+
+	private ObservableCollection<Issue33688Item> _items = new();
+
+	public ObservableCollection<Issue33688Item> Items
+	{
+		get => _items;
+		set
+		{
+			if (_items != value)
+			{
+				_items = value;
+				OnPropertyChanged();
+			}
+		}
+	}
+
+	public ICommand SaveAndNavigateBackCommand { get; }
+	public ICommand FilterCommand { get; }
+
+	public Issue33688ViewModel()
+	{
+		SaveAndNavigateBackCommand = new Command(() =>
+		{
+			// Update the result label, then navigate back
+			if (_resultLabelRef != null)
+			{
+				_resultLabelRef.Text = "BackButtonBehavior triggered!";
+			}
+			Shell.Current.GoToAsync("..");
+		});
+
+		FilterCommand = new Command(() =>
+		{
+			// This is the key action that triggers the bug:
+			// Setting Items to a new ObservableCollection AFTER the page is displayed
+			Items = new ObservableCollection<Issue33688Item>
+			{
+				new Issue33688Item { Name = "Item 1" },
+				new Issue33688Item { Name = "Item 2" },
+				new Issue33688Item { Name = "Item 3" }
+			};
+		});
+	}
+
+	public event PropertyChangedEventHandler PropertyChanged = null!;
+
+	protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null!)
+	{
+		PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+	}
+}
+
+file class Issue33688Item
+{
+	public string Name { get; set; } = string.Empty;
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue33688.cs

@@ -0,0 +1,44 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue33688 : _IssuesUITest
+{
+	public override string Issue => "BackButtonBehavior is no longer triggered once a ContentPage contains a CollectionView and the ItemsSource has been changed";
+
+	public Issue33688(TestDevice device) : base(device) { }
+
+	[Test]
+	[Category(UITestCategories.Shell)]
+	public void BackButtonBehaviorTriggersWithCollectionView()
+	{
+		// Wait for main page to load
+		App.WaitForElement("NavigateButton");
+
+		// Navigate to the second page with CollectionView
+		App.Tap("NavigateButton");
+
+		// Wait for the second page to load - use StatusLabel as primary indicator
+		App.WaitForElement("StatusLabel");
+
+		// Find and tap the filter button to load items - this triggers the bug
+		// (setting ItemsSource to a new ObservableCollection)
+		App.WaitForElement("FilterButton");
+		App.Tap("FilterButton");
+
+		// Give time for the CollectionView to update
+		App.WaitForElement("TestCollectionView");
+
+		// Press the back button
+		App.TapBackArrow();
+
+		// Wait for navigation back and verify BackButtonBehavior was triggered
+		App.WaitForElement("ResultLabel");
+
+		var resultText = App.FindElement("ResultLabel").GetText();
+		Assert.That(resultText, Is.EqualTo("BackButtonBehavior triggered!"), 
+			"BackButtonBehavior command should have been executed when pressing back button");
+	}
+}