[Android] - Fix KeepScrollOffset Behavior During Dynamic Item Additions in CollectionView

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

@@ -370,10 +370,9 @@ public virtual void UpdateItemsSource()
 			UpdateAdapter();
 
 			// Set up any properties which require observing data changes in the adapter
-			UpdateItemsUpdatingScrollMode();
-
 			UpdateEmptyView();
 			AddOrUpdateScrollListener();
+			UpdateItemsUpdatingScrollMode();
 			UpdateSnapBehavior();
 		}
 
@@ -382,6 +381,15 @@ protected virtual void UpdateItemsUpdatingScrollMode()
 			if (ItemsViewAdapter == null || ItemsView == null)
 				return;
 
+			if (ItemsView.ItemsUpdatingScrollMode == ItemsUpdatingScrollMode.KeepScrollOffset)
+			{
+				ScrollHelper.AddScrollListener();
+			}
+			else
+			{
+				ScrollHelper.RemoveScrollListener();
+			}
+
 			if (ItemsView.ItemsUpdatingScrollMode == ItemsUpdatingScrollMode.KeepItemsInView)
 			{
 				// Keeping the current items in view is the default, so we don't need to watch for data changes

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

@@ -12,7 +12,7 @@ internal class ScrollHelper : RecyclerView.OnScrollListener
 
 		bool _undoNextScrollAdjustment;
 		bool _maintainingScrollOffsets;
-
+		bool _isAtScrollOrigin = true;
 		int _lastScrollX;
 		int _lastScrollY;
 		int _lastDeltaX;
@@ -26,13 +26,6 @@ public ScrollHelper(RecyclerView recyclerView)
 		// Used by the renderer to maintain scroll offset when using ItemsUpdatingScrollMode KeepScrollOffset
 		public void UndoNextScrollAdjustment()
 		{
-			// Don't start tracking the scroll offsets until we really need to
-			if (!_maintainingScrollOffsets)
-			{
-				_maintainingScrollOffsets = true;
-				_recyclerView.AddOnScrollListener(this);
-			}
-
 			_undoNextScrollAdjustment = true;
 
 			_lastScrollX = _recyclerView.ComputeHorizontalScrollOffset();
@@ -212,18 +205,46 @@ void TrackOffsets()
 				// offset to shift; since the ItemsUpdatingScrollMode is set to KeepScrollOffset; we need to undo 
 				// that shift and stay where we were before the item was added
 
-				_undoNextScrollAdjustment = false;
-				_recyclerView.ScrollBy(-_lastDeltaX, -_lastDeltaY);
+				if (_isAtScrollOrigin)
+				{
+					_recyclerView.ScrollBy(-_lastDeltaX, -_lastDeltaY);
+				}
 
+				_undoNextScrollAdjustment = false;
 				_lastDeltaX = 0;
 				_lastDeltaY = 0;
 			}
+			else
+			{
+				_isAtScrollOrigin = newXOffset == 0
+					&& newYOffset == 0;
+			}
 		}
 
 		public override void OnScrolled(RecyclerView recyclerView, int dx, int dy)
 		{
 			base.OnScrolled(recyclerView, dx, dy);
 			TrackOffsets();
 		}
+
+		internal void AddScrollListener()
+		{
+			// Set up scroll listener to track the scroll offsets when we're using KeepScrollOffset.
+			if (!_maintainingScrollOffsets)
+			{
+				_maintainingScrollOffsets = true;
+				_recyclerView.AddOnScrollListener(this);
+			}
+		}
+
+		internal void RemoveScrollListener()
+		{
+			// Remove the scroll listener when we're done and no longer need to track the offsets.
+			if (_maintainingScrollOffsets)
+			{
+				_maintainingScrollOffsets = false;
+				_recyclerView.RemoveOnScrollListener(this);
+			}
+		}
 	}
 }

src/Controls/tests/TestCases.HostApp/Issues/Issue29131.cs

@@ -0,0 +1,85 @@
+using System.Collections.ObjectModel;
+using Maui.Controls.Sample.Issues;
+
+namespace Controls.TestCases.HostApp.Issues;
+
+[Issue(IssueTracker.Github, 29131, "Android - KeepScrollOffset does not work as expected when new items are added in CollectionView", PlatformAffected.Android)]
+public class Issue29131 : TestContentPage
+{
+	ObservableCollection<string> items;
+	CollectionView collectionView;
+	int count = 1;
+
+	protected override void Init()
+	{
+		items = new ObservableCollection<string>(Enumerable.Range(1, 30).Select(i => $"Item {i}"));
+
+		Button keepScrollOffsetButton = CreateButton("KeepScrollOffset", "KeepScrollOffsetButton", OnKeepScrollOffsetClicked);
+		Button addButton = CreateButton("Add Item to Top", "AddNewItem", OnAddItemClicked);
+		Button scrollButton = CreateButton("Scroll CollectionView", "ScrollButton", OnScrollButtonClicked);
+
+		collectionView = new CollectionView
+		{
+			AutomationId = "CollectionView",
+			ItemsSource = items,
+			ItemTemplate = new DataTemplate(() =>
+			{
+				var label = new Label();
+				label.SetBinding(Label.TextProperty, ".");
+				return new Border
+				{
+					Content = label,
+					Padding = 10,
+					Margin = new Thickness(5),
+					BackgroundColor = Colors.LightGray,
+				};
+			})
+		};
+
+		Grid grid = new Grid
+		{
+			RowDefinitions =
+			{
+				new RowDefinition { Height = GridLength.Auto },
+				new RowDefinition { Height = GridLength.Auto },
+				new RowDefinition { Height = GridLength.Auto },
+				new RowDefinition { Height = GridLength.Star }
+			},
+			RowSpacing = 5
+		};
+		grid.Add(keepScrollOffsetButton, 0, 0);
+		grid.Add(addButton, 0, 1);
+		grid.Add(scrollButton, 0, 2);
+		grid.Add(collectionView, 0, 3);
+
+		Content = grid;
+	}
+
+	Button CreateButton(string text, string automationId, EventHandler onClick)
+	{
+		return new Button
+		{
+			Text = text,
+			AutomationId = automationId,
+			Command = new Command(_ => onClick(this, EventArgs.Empty))
+		};
+	}
+
+	void OnKeepScrollOffsetClicked(object sender, EventArgs e)
+	{
+		collectionView.ItemsUpdatingScrollMode = ItemsUpdatingScrollMode.KeepScrollOffset;
+	}
+
+	void OnScrollButtonClicked(object sender, EventArgs e)
+	{
+		int index = (count % 2 == 0) ? 0 : items.Count - 1;
+		var position = (count % 2 == 0) ? ScrollToPosition.Start : ScrollToPosition.End;
+		collectionView.ScrollTo(index, position: position, animate: true);
+		count++;
+	}
+
+	void OnAddItemClicked(object sender, EventArgs e)
+	{
+		items.Insert(0, $"Item {items.Count + 1}");
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/CollectionViewUITests.CollectionViewItemsUpdatingScrollMode.cs

@@ -36,6 +36,10 @@ public void KeepItemsInView()
 
 #if TEST_FAILS_ON_IOS && TEST_FAILS_ON_CATALYST // The test fails on iOS and macOS because Appium is unable to locate the Picker control elements resulting in a TimeoutException. For more information, see: https://github.com/dotnet/maui/issues/28024
 		// KeepScrollOffset (src\Compatibility\ControlGallery\src\Issues.Shared\CollectionViewItemsUpdatingScrollMode.cs)
+		// After scrolling to the middle, adding items above should not cause the view to scroll.
+		// Previously, the view would automatically scroll to show newly added items, which broke
+		// KeepScrollOffset semantics. With the fix, the visible items remain stable; adding 5 items
+		// above shifts indices so the previously visible "Vegetables.jpg, 10" becomes "FlowerBuds.jpg, 12".
 		[Test]
 		[Category(UITestCategories.CollectionView)]
 		public void KeepScrollOffset()
@@ -47,8 +51,13 @@ public void KeepScrollOffset()
 			App.WaitForElement("ScrollToMiddle");
 			App.Click("ScrollToMiddle");
 			App.WaitForElement("Vegetables.jpg, 10");
-			App.Click("AddItemAbove");
-			App.WaitForElement("photo.jpg, 9");
+
+			for (int i = 0; i < 5; i++)
+			{
+				App.Click("AddItemAbove");
+			}
+
+			App.WaitForElement("FlowerBuds.jpg, 12");
 		}
 
 		// KeepLastItemInView(src\Compatibility\ControlGallery\src\Issues.Shared\CollectionViewItemsUpdatingScrollMode.cs)

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue29131.cs

@@ -0,0 +1,31 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue29131 : _IssuesUITest
+{
+	public Issue29131(TestDevice device) : base(device) { }
+
+	public override string Issue => "Android - KeepScrollOffset does not work as expected when new items are added in CollectionView";
+	const string AddNewItem = "AddNewItem";
+	const string ScrollButton = "ScrollButton";
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void KeepScrollOffsetShouldWork()
+	{
+		App.WaitForElement("CollectionView");
+		App.Click("KeepScrollOffsetButton");
+		App.Click(ScrollButton);
+		App.Click(AddNewItem);
+		App.WaitForElement("Item 30");
+		App.Click(ScrollButton);
+		App.Click(AddNewItem);
+		App.WaitForElement("Item 32");
+		App.Click(ScrollButton);
+		App.Click(AddNewItem);
+		App.WaitForElement("Item 30");
+	}
+}