Fix VerticalOffset Update When Modifying CollectionView.ItemsSource While Scrolled

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

@@ -34,12 +34,9 @@ public override void OnScrolled(RecyclerView recyclerView, int dx, int dy)
 		{
 			base.OnScrolled(recyclerView, dx, dy);
 
-			// TODO: These offsets will be incorrect upon row size or count change.
-			// They are currently provided in place of LayoutManager's default offset calculation
-			// because it does not report accurate values in the presence of uneven rows.
-			// See https://stackoverflow.com/questions/27507715/android-how-to-get-the-current-x-offset-of-recyclerview
-			_horizontalOffset += dx;
-			_verticalOffset += dy;
+			var itemCount = recyclerView.GetAdapter()?.ItemCount;
+			_horizontalOffset = itemCount == 0 ? 0 : _horizontalOffset + dx;
+			_verticalOffset = itemCount == 0 ? 0 : _verticalOffset + dy;
 
 			var (First, Center, Last) = GetVisibleItemsIndex(recyclerView);
 			var itemsViewScrolledEventArgs = new ItemsViewScrolledEventArgs

src/Controls/src/Core/Handlers/Items/iOS/ItemsViewDelegator.cs

@@ -29,12 +29,9 @@ public override void Scrolled(UIScrollView scrollView)
 		{
 			var (visibleItems, firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex) = GetVisibleItemsIndex();
 
-			if (!visibleItems)
-				return;
-
 			var contentInset = scrollView.ContentInset;
-			var contentOffsetX = scrollView.ContentOffset.X + contentInset.Left;
-			var contentOffsetY = scrollView.ContentOffset.Y + contentInset.Top;
+			var contentOffsetX = !visibleItems ? 0 : scrollView.ContentOffset.X + contentInset.Left;
+			var contentOffsetY = !visibleItems ? 0 : scrollView.ContentOffset.Y + contentInset.Top;
 
 			var itemsViewScrolledEventArgs = new ItemsViewScrolledEventArgs
 			{

src/Controls/src/Core/Handlers/Items2/iOS/ItemsViewDelegator2.cs

@@ -30,12 +30,9 @@ public override void Scrolled(UIScrollView scrollView)
 		{
 			var (visibleItems, firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex) = GetVisibleItemsIndex();
 
-			if (!visibleItems)
-				return;
-
 			var contentInset = scrollView.ContentInset;
-			var contentOffsetX = scrollView.ContentOffset.X + contentInset.Left;
-			var contentOffsetY = scrollView.ContentOffset.Y + contentInset.Top;
+			var contentOffsetX = !visibleItems ? 0 : scrollView.ContentOffset.X + contentInset.Left;
+			var contentOffsetY = !visibleItems ? 0 : scrollView.ContentOffset.Y + contentInset.Top;
 
 			var itemsViewScrolledEventArgs = new ItemsViewScrolledEventArgs
 			{

src/Controls/tests/TestCases.HostApp/Issues/Issue21708.xaml

@@ -0,0 +1,69 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+			       xmlns:controls="clr-namespace:Maui.Controls.Sample.Issues"
+             x:Class="Maui.Controls.Sample.Issues.Issue21708">
+
+	<Grid>
+        <Grid.RowDefinitions>
+            <RowDefinition Height="Auto" />
+            <RowDefinition Height="Auto" />
+            <RowDefinition />
+        </Grid.RowDefinitions>
+        <Grid.ColumnDefinitions>
+            <ColumnDefinition />
+            <ColumnDefinition />
+			<ColumnDefinition />
+        </Grid.ColumnDefinitions>
+
+        <Button
+            Grid.Row="0"
+            Grid.Column="0"
+            Margin="10"
+			AutomationId="Fill"
+            Clicked="FillButton_OnClicked"
+            Text="Fill" />
+
+        <Button
+            Grid.Row="0"
+            Grid.Column="2"
+            Margin="10"
+			AutomationId="Empty"
+            Clicked="EmptyButton_OnClicked"
+            Text="Empty" />
+
+                   <Label
+        Grid.Row="1"
+        Grid.Column="0"
+        AutomationId="VerticalOffsetLabel"
+        Text="VerticalOffset: " />
+
+    <Label AutomationId="Label"
+        Grid.ColumnSpan="2"
+        Grid.Row="1"
+        HorizontalTextAlignment="Start"
+        Grid.Column="1"
+        Text="{Binding VerticalOffset}" />
+
+        <CollectionView AutomationId="CollectionView"
+            x:Name="CollectionView"
+            Grid.Row="2"
+            Grid.Column="0"
+            Grid.ColumnSpan="3"
+            ItemsSource="{Binding Items}"
+            Scrolled="CollectionView_OnScrolled">
+            <CollectionView.ItemTemplate>
+                <DataTemplate>
+                    <Label
+                        Margin="10"
+                        FontSize="Default"
+                        HeightRequest="20"
+                        HorizontalTextAlignment="Center"
+                        Text="{Binding .}"
+                        TextColor="RoyalBlue" />
+                </DataTemplate>
+            </CollectionView.ItemTemplate>
+        </CollectionView>
+    </Grid>
+
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue21708.xaml.cs

@@ -0,0 +1,52 @@
+using System.Collections.ObjectModel;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.Github, 21708, "CollectionView.Scrolled event offset isn't correctly reset when items change", PlatformAffected.All)]
+	public partial class Issue21708 : ContentPage
+	{
+		ObservableCollection<int> _items;
+        double _verticalOffset;
+
+        public Issue21708()
+        {
+            InitializeComponent();
+            _items = new ObservableCollection<int>();
+            AddItemsToCollectionView();
+            CollectionView.ItemsSource = _items;
+			BindingContext = this;
+        }
+
+        public double VerticalOffset
+        {
+            get => _verticalOffset;
+            set
+            {
+                _verticalOffset = value;
+                OnPropertyChanged(nameof(VerticalOffset));
+            }
+        }
+
+        void CollectionView_OnScrolled(object sender, ItemsViewScrolledEventArgs e)
+        {
+            VerticalOffset = e.VerticalOffset;
+        }
+
+        void EmptyButton_OnClicked(object sender, EventArgs e)
+        {
+            _items.Clear();
+        }
+
+        void FillButton_OnClicked(object sender, EventArgs e)
+		{
+			AddItemsToCollectionView();
+		}
+
+		void AddItemsToCollectionView()
+		{
+			foreach (var i in Enumerable.Range(0, 50))
+				_items.Add(i);
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue21708.cs

@@ -0,0 +1,23 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue21708 : _IssuesUITest
+{
+	public Issue21708(TestDevice device) : base(device) { }
+
+	public override string Issue => "CollectionView.Scrolled event offset isn't correctly reset when items change";
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void VerifyCollectionViewVerticalOffset()
+	{
+		App.WaitForElement("Fill");
+		App.ScrollDown("CollectionView");
+		Assert.That(App.FindElement("Label").GetText(), Is.GreaterThan("0"));
+		App.Tap("Empty");
+		Assert.That(App.FindElement("Label").GetText(), Is.EqualTo("0"));
+	}
+}