[Android] Fix AppBar flicker on CheckBox/Switch toggle with Material 3

src/Controls/src/Core/Handlers/Items/Android/MauiRecyclerView.cs

@@ -8,8 +8,10 @@
 using AndroidX.RecyclerView.Widget;
 using Microsoft.Maui.Controls.Internals;
 using Microsoft.Maui.Controls.Platform;
+using Microsoft.Maui.Platform;
 using Microsoft.Maui.Graphics;
 using ARect = Android.Graphics.Rect;
+using AView = Android.Views.View;
 using AViewCompat = AndroidX.Core.View.ViewCompat;
 
 namespace Microsoft.Maui.Controls.Handlers.Items
@@ -47,6 +49,7 @@ public class MauiRecyclerView<TItemsView, TAdapter, TItemsViewSource> : Recycler
 		SimpleItemTouchHelperCallback _itemTouchHelperCallback;
 		WeakNotifyPropertyChangedProxy _layoutPropertyChangedProxy;
 		PropertyChangedEventHandler _layoutPropertyChanged;
+		Java.Lang.IRunnable _setAppBarLiftTargetRunnable;
 
 		~MauiRecyclerView() => _layoutPropertyChangedProxy?.Unsubscribe();
 
@@ -59,6 +62,73 @@ public MauiRecyclerView(Context context, Func<IItemsLayout> getItemsLayout, Func
 			_itemsUpdateScrollObserver = new DataChangeObserver(AdjustScrollForItemUpdate);
 		}
 
+		protected override void OnAttachedToWindow()
+		{
+			base.OnAttachedToWindow();
+
+			if (RuntimeFeature.IsMaterial3Enabled)
+			{
+				PostTrySetAppBarLiftTargetIfOnScreen();
+			}
+		}
+
+		protected override void OnDetachedFromWindow()
+		{
+			// Clean up AppBar listener while the ViewTreeObserver is still valid.
+			if (RuntimeFeature.IsMaterial3Enabled)
+			{
+				ClearAppBarLiftTargetAndPendingPost();
+			}
+
+			base.OnDetachedFromWindow();
+		}
+
+		protected override void OnVisibilityChanged(AView changedView, ViewStates visibility)
+		{
+			base.OnVisibilityChanged(changedView, visibility);
+
+			if (changedView != this)
+			{
+				return;
+			}
+
+			if (!RuntimeFeature.IsMaterial3Enabled)
+			{
+				return;
+			}
+
+			if (visibility == ViewStates.Visible)
+			{
+				PostTrySetAppBarLiftTargetIfOnScreen();
+			}
+			else
+			{
+				ClearAppBarLiftTargetAndPendingPost();
+			}
+		}
+
+		void PostTrySetAppBarLiftTargetIfOnScreen()
+		{
+			var runnable = GetOrCreateSetAppBarLiftTargetRunnable();
+			RemoveCallbacks(runnable);
+			Post(runnable);
+		}
+
+		void ClearAppBarLiftTargetAndPendingPost()
+		{
+			if (_setAppBarLiftTargetRunnable is not null)
+			{
+				RemoveCallbacks(_setAppBarLiftTargetRunnable);
+			}
+
+			this.ClearAppBarLiftTarget();
+		}
+
+		Java.Lang.IRunnable GetOrCreateSetAppBarLiftTargetRunnable()
+		{
+			return _setAppBarLiftTargetRunnable ??= new Java.Lang.Runnable(() => this.TrySetAppBarLiftTargetIfOnScreen());
+		}
+
 		public virtual void TearDownOldElement(TItemsView oldElement)
 		{
 			// Stop listening for layout property changes

src/Controls/src/Core/PublicAPI/net-android/PublicAPI.Unshipped.txt

@@ -13,6 +13,10 @@ override Microsoft.Maui.Controls.Shapes.Shape.OnPropertyChanged(string? property
 ~override Microsoft.Maui.Controls.Handlers.Items.MauiRecyclerView<TItemsView, TAdapter, TItemsViewSource>.OnTouchEvent(Android.Views.MotionEvent e) -> bool
 ~override Microsoft.Maui.Controls.Handlers.Items.RecyclerViewScrollListener<TItemsView, TItemsViewSource>.OnScrollStateChanged(AndroidX.RecyclerView.Widget.RecyclerView recyclerView, int newState) -> void
 ~override Microsoft.Maui.Controls.Handlers.Items.SelectableItemsViewAdapter<TItemsView, TItemsSource>.IsSelectionEnabled(Android.Views.ViewGroup parent, int viewType) -> bool
+override Microsoft.Maui.Controls.Handlers.Items.MauiRecyclerView<TItemsView, TAdapter, TItemsViewSource>.OnAttachedToWindow() -> void
+override Microsoft.Maui.Controls.Handlers.Items.MauiRecyclerView<TItemsView, TAdapter, TItemsViewSource>.OnDetachedFromWindow() -> void
+override Microsoft.Maui.Controls.TitleBar.OnBindingContextChanged() -> void
+~override Microsoft.Maui.Controls.Handlers.Items.MauiRecyclerView<TItemsView, TAdapter, TItemsViewSource>.OnVisibilityChanged(Android.Views.View changedView, Android.Views.ViewStates visibility) -> void
 override Microsoft.Maui.Controls.Platform.Compatibility.ShellSectionRenderer.OnHiddenChanged(bool hidden) -> void
 override Microsoft.Maui.Controls.SwipeItemView.IsEnabledCore.get -> bool
 ~override Microsoft.Maui.Controls.RadioButton.OnPropertyChanged(string propertyName = null) -> void

src/Core/src/Platform/Android/AppbarLayoutExtensions.cs

@@ -0,0 +1,263 @@
+using System;
+using System.Runtime.CompilerServices;
+using Android.Graphics;
+using Android.Views;
+using Google.Android.Material.AppBar;
+
+namespace Microsoft.Maui.Platform
+{
+    // Manages pinning the lift-on-scroll target of a MAUI navigation AppBarLayout
+    // to a specific scrollable view (e.g. MauiScrollView or RecyclerView).
+    // Shared between MauiScrollView and MauiRecyclerView to avoid duplicating
+    // the same ancestor-walk / attach / detach logic.
+    //
+    // When the scrollable view is inside a CarouselView/ViewPager2, adjacent
+    // off-screen pages are pre-cached and their views stay attached without
+    // receiving visibility callbacks during page swipes.  A ViewTreeObserver
+    // scroll-changed listener detects these transitions and transfers the
+    // lift target to whichever page's scrollable view is currently on-screen.
+    //
+    // Per-view state is stored in a ConditionalWeakTable so that it is
+    // automatically cleaned up when the View is garbage-collected.
+    internal static class AppbarLayoutExtensions
+    {
+        static readonly ConditionalWeakTable<View, AppBarLiftState> s_stateTable = new();
+
+        sealed class AppBarLiftState
+        {
+            public AppBarLayout? LiftOnScrollAppBar;
+            public ScrollChangedListener? ScrollListener;
+            public ViewTreeObserver? ScrollListenerObserver;
+            public readonly Rect VisibleRect = new();
+        }
+
+        internal static void TrySetAppBarLiftTargetIfOnScreen(this View view)
+        {
+            // Guard: the view may have detached or been hidden between Post() and execution.
+            if (!view.IsAlive() || !view.IsAttachedToWindow || view.Visibility != ViewStates.Visible)
+            {
+                return;
+            }
+
+            var state = s_stateTable.GetOrCreateValue(view);
+
+            // Single ancestor walk up front — bail early if no AppBarLayout exists.
+            // This avoids registering a ViewTreeObserver listener on pages without an app bar.
+            var appBar = FindAppBarLayout(view, out bool hasAncestorScrollView);
+            if (hasAncestorScrollView || appBar is null)
+            {
+                return;
+            }
+
+            // When inside a CarouselView, ViewPager2 pre-loads adjacent off-screen pages,
+            // so their ScrollViews also attach. Only the on-screen page's ScrollView should
+            // claim the lift target. GetGlobalVisibleRect returns false if the view is
+            // entirely outside the clipped viewport (e.g. a pre-loaded carousel page).
+            if (view.GetGlobalVisibleRect(state.VisibleRect))
+            {
+                SetAppBarLiftTarget(view, state, appBar);
+            }
+
+            // Listen for parent scroll changes to detect carousel page transitions.
+            // We only reach here when an AppBarLayout was found above.
+            StartListeningForParentScrollChanges(view, state);
+        }
+
+        internal static void ClearAppBarLiftTarget(this View view)
+        {
+            if (!s_stateTable.TryGetValue(view, out var state))
+            {
+                return;
+            }
+
+            StopListeningForParentScrollChanges(view, state);
+            ClearAppBarLiftTargetCore(view, state);
+        }
+
+        static void ClearAppBarLiftTargetCore(View view, AppBarLiftState state)
+        {
+            if (state.LiftOnScrollAppBar is null)
+            {
+                return;
+            }
+
+            // Only clear if we're still the current target; avoid stomping on another scroll view
+            // that may have been set as the target after us.
+            if (state.LiftOnScrollAppBar.LiftOnScrollTargetViewId == view.Id)
+            {
+                state.LiftOnScrollAppBar.LiftOnScrollTargetViewId = View.NoId;
+            }
+
+            state.LiftOnScrollAppBar = null;
+        }
+
+        static void TrySetAppBarLiftTarget(View view, AppBarLiftState state)
+        {
+            // Ancestor walk to find the AppBarLayout while also checking
+            // whether a MauiScrollView ancestor exists (which should own the
+            // lift target instead of this view).
+            var appBar = FindAppBarLayout(view, out bool hasAncestorScrollView);
+            if (hasAncestorScrollView || appBar is null)
+            {
+                return;
+            }
+
+            SetAppBarLiftTarget(view, state, appBar);
+        }
+
+        static void SetAppBarLiftTarget(View view, AppBarLiftState state, AppBarLayout appBar)
+        {
+            if (view.Id == View.NoId)
+            {
+                // LiftOnScrollTargetViewId requires a non-NoId view id.
+                // Intentionally assigning a generated id here; the view will
+                // keep this id for the rest of its lifetime, which is fine
+                // because MauiScrollView / MauiRecyclerView are not looked up
+                // by id by any other host code.
+                view.Id = View.GenerateViewId();
+            }
+
+            state.LiftOnScrollAppBar = appBar;
+            appBar.LiftOnScrollTargetViewId = view.Id;
+
+            // Force the AppBar to reflect this view's current scroll position.
+            // After a carousel swipe or visibility toggle the AppBar may be stuck
+            // in the wrong state; CanScrollVertically(-1) is true when the view
+            // has been scrolled down from the top.
+            appBar.SetLifted(view.CanScrollVertically(-1));
+        }
+
+        static void OnParentScrollChanged(View view, AppBarLiftState state)
+        {
+            if (!view.IsAlive() || !view.IsAttachedToWindow || view.Visibility != ViewStates.Visible)
+            {
+                return;
+            }
+
+            bool isOnScreen = view.GetGlobalVisibleRect(state.VisibleRect);
+            bool ownsTarget = state.LiftOnScrollAppBar is not null;
+
+            if (isOnScreen && !ownsTarget)
+            {
+                TrySetAppBarLiftTarget(view, state);
+            }
+            else if (!isOnScreen && ownsTarget)
+            {
+                // Release without stopping the listener — we still need to
+                // detect when the carousel swipes back to this page.
+                ClearAppBarLiftTargetCore(view, state);
+            }
+        }
+
+        static void StartListeningForParentScrollChanges(View view, AppBarLiftState state)
+        {
+            if (state.ScrollListener is not null)
+            {
+                return;
+            }
+
+            var observer = view.ViewTreeObserver;
+            if (observer is null || !observer.IsAlive)
+            {
+                return;
+            }
+
+            state.ScrollListener = new ScrollChangedListener(view, state);
+            state.ScrollListenerObserver = observer;
+            observer.AddOnScrollChangedListener(state.ScrollListener);
+        }
+
+        static void StopListeningForParentScrollChanges(View view, AppBarLiftState state)
+        {
+            if (state.ScrollListener is null)
+            {
+                return;
+            }
+
+            // Remove from the same observer instance used when adding.
+            var observer = state.ScrollListenerObserver;
+            if (observer is not null && observer.IsAlive)
+            {
+                observer.RemoveOnScrollChangedListener(state.ScrollListener);
+            }
+            else
+            {
+                // Fallback for safety if observer rotated between add/remove.
+                observer = view.ViewTreeObserver;
+                if (observer is not null && observer.IsAlive)
+                {
+                    observer.RemoveOnScrollChangedListener(state.ScrollListener);
+                }
+            }
+
+            state.ScrollListener.Dispose();
+            state.ScrollListener = null;
+            state.ScrollListenerObserver = null;
+        }
+
+        static AppBarLayout? FindAppBarLayout(View view, out bool hasAncestorScrollView)
+        {
+            // Single ancestor walk that both checks for a MauiScrollView ancestor
+            // (which should own the lift target instead) AND finds the AppBarLayout.
+            // NavigationPage uses Resource.Id.navigationlayout_appbar, but Shell creates
+            // its AppBarLayout programmatically without an ID, so we match any AppBarLayout.
+            //
+            // Nested-host note: Shell-inside-NavigationPage (or vice versa) is not a
+            // supported MAUI configuration, so there is only ever one relevant AppBarLayout
+            // in the ancestor/sibling chain for any given scroll view.  The walk returns the
+            // first one found, which is the correct one for all supported layouts.
+            hasAncestorScrollView = false;
+            var parent = view.Parent;
+
+            while (parent is View parentView)
+            {
+                if (parentView is MauiScrollView)
+                {
+                    hasAncestorScrollView = true;
+                    return null;
+                }
+
+                // Stop the MauiScrollView check once we reach the AppBarLayout level —
+                // anything above that isn't "inside the page".
+                if (parentView is AppBarLayout directAppBar)
+                {
+                    return directAppBar;
+                }
+
+                if (parentView is ViewGroup group)
+                {
+                    for (int i = 0; i < group.ChildCount; i++)
+                    {
+                        if (group.GetChildAt(i) is AppBarLayout siblingAppBar)
+                        {
+                            return siblingAppBar;
+                        }
+                    }
+                }
+
+                parent = parentView.Parent;
+            }
+
+            return null;
+        }
+
+        // Lightweight Java-side listener that forwards ViewTreeObserver scroll
+        // changes back to the static extension for carousel page-change detection.
+        sealed class ScrollChangedListener : Java.Lang.Object, ViewTreeObserver.IOnScrollChangedListener
+        {
+            readonly View _view;
+            readonly AppBarLiftState _state;
+
+            public ScrollChangedListener(View view, AppBarLiftState state)
+            {
+                _view = view;
+                _state = state;
+            }
+
+            public void OnScrollChanged()
+            {
+                OnParentScrollChanged(_view, _state);
+            }
+        }
+    }
+}