[iOS/Mac] CollectionView: Fix incorrect ItemsViewScrolledEventArgs indices with grouped items

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

@@ -117,7 +117,8 @@ protected virtual (bool VisibleItems, NSIndexPath First, NSIndexPath Center, NSI
 			if (collectionView is null)
 				return default;
 
-			var indexPathsForVisibleItems = collectionView.IndexPathsForVisibleItems.OrderBy(x => x.Row).ToList();
+			// Sort visible item index paths by section and then by row for consistent order in both grouped and ungrouped sources
+			var indexPathsForVisibleItems = collectionView.IndexPathsForVisibleItems.OrderBy(x => x.Section).ThenBy(x => x.Row).ToList();
 
 			var visibleItems = indexPathsForVisibleItems.Count > 0;
 			NSIndexPath firstVisibleItemIndex = null, centerItemIndex = null, lastVisibleItemIndex = null;

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

@@ -118,7 +118,8 @@ protected virtual (bool VisibleItems, NSIndexPath First, NSIndexPath Center, NSI
 			if (collectionView is null)
 				return default;
 
-			var indexPathsForVisibleItems = collectionView.IndexPathsForVisibleItems.OrderBy(x => x.Row).ToList();
+			// Sort visible item index paths by section and then by row for consistent order in both grouped and ungrouped sources
+			var indexPathsForVisibleItems = collectionView.IndexPathsForVisibleItems.OrderBy(x => x.Section).ThenBy(x => x.Row).ToList();
 
 			var visibleItems = indexPathsForVisibleItems.Count > 0;
 			NSIndexPath firstVisibleItemIndex = null, centerItemIndex = null, lastVisibleItemIndex = null;

src/Controls/tests/TestCases.HostApp/Issues/Issue17664.cs

@@ -0,0 +1,120 @@
+using System.Collections.ObjectModel;
+
+namespace Maui.Controls.Sample.Issues;
+
+[Issue(IssueTracker.Github, 17664, "Incorrect ItemsViewScrolledEventArgs in CollectionView when IsGrouped is set to true", PlatformAffected.iOS)]
+public class Issue17664 : ContentPage
+{
+	CollectionView _collectionView;
+	Label descriptionLabel;
+	ObservableCollection<Issue17664_ItemModelGroup> _groupedItems;
+
+	public Issue17664()
+	{
+		Button scrollButton = new Button
+		{
+			AutomationId = "Issue17664ScrollBtn",
+			Text = "Scroll to Category C, Item #2"
+		};
+		scrollButton.Clicked += ScrollButton_Clicked;
+
+		descriptionLabel = new Label
+		{
+			AutomationId = "Issue17664DescriptionLabel",
+			Text = "Use the button above to scroll the CollectionView.",
+			FontSize = 14,
+			HorizontalOptions = LayoutOptions.Center
+		};
+
+		_collectionView = new CollectionView
+		{
+			IsGrouped = true,
+			GroupHeaderTemplate = new DataTemplate(() =>
+			{
+				Label label = new Label
+				{
+					FontAttributes = FontAttributes.Bold,
+					BackgroundColor = Colors.LightGray,
+					Padding = 10
+				};
+
+				label.SetBinding(Label.TextProperty, "Name");
+				return label;
+			}),
+			ItemTemplate = new DataTemplate(() =>
+			{
+				Label textLabel = new Label
+				{
+					FontAttributes = FontAttributes.Bold,
+					Padding = 30
+				};
+
+				textLabel.SetBinding(Label.TextProperty, ".");
+				return textLabel;
+			})
+		};
+
+		_collectionView.Scrolled += (s, e) =>
+		{
+			var flatItems = _groupedItems.SelectMany(group => group).ToList();
+			if (e.LastVisibleItemIndex >= 0 && e.LastVisibleItemIndex < flatItems.Count)
+			{
+				descriptionLabel.Text = flatItems[e.LastVisibleItemIndex];
+			}
+		};
+
+		List<string> categories = new List<string> { "Category A", "Category B", "Category C" };
+
+		_groupedItems = new ObservableCollection<Issue17664_ItemModelGroup>();
+
+		foreach (var category in categories)
+		{
+			List<string> items = new List<string>();
+
+			for (int i = 0; i < 5; i++)
+			{
+				items.Add($"{category} item #{i}");
+			}
+
+			_groupedItems.Add(new Issue17664_ItemModelGroup(category, items));
+		}
+
+		_collectionView.ItemsSource = _groupedItems;
+
+		Grid grid = new Grid
+		{
+			RowSpacing = 10,
+			Padding = 10,
+			RowDefinitions =
+			{
+				new RowDefinition { Height = GridLength.Auto },
+				new RowDefinition { Height = GridLength.Auto },
+				new RowDefinition { Height = GridLength.Star }
+			}
+		};
+
+		grid.Add(scrollButton, 0, 0);
+		grid.Add(descriptionLabel, 0, 1);
+		grid.Add(_collectionView, 0, 2);
+
+		Content = grid;
+	}
+
+	private void ScrollButton_Clicked(object sender, EventArgs e)
+	{
+		var targetGroup = _groupedItems.FirstOrDefault(group => group.Name == "Category C");
+		var targetItem = targetGroup.FirstOrDefault(item => item == "Category C item #2");
+
+		_collectionView.ScrollTo(targetItem, targetGroup, ScrollToPosition.End);
+	}
+}
+
+public class Issue17664_ItemModelGroup : ObservableCollection<string>
+{
+	public string Name { get; set; }
+
+	public Issue17664_ItemModelGroup(string name, IEnumerable<string> items) : base(items)
+	{
+		Name = name;
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue17664.cs

@@ -0,0 +1,30 @@
+#if TEST_FAILS_ON_ANDROID && TEST_FAILS_ON_WINDOWS // Android fix: https://github.com/dotnet/maui/pull/31437
+// Windows: The Scrolled event is not consistently triggered in the CI environment during automated
+// scrolling, so the label text is never updated. This is a test infrastructure limitation on Windows;
+// the fix itself is iOS/MacCatalyst-only and works correctly on iOS and MacCatalyst.
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue17664 : _IssuesUITest
+{
+	public Issue17664(TestDevice device) : base(device)
+	{
+	}
+
+	public override string Issue => "Incorrect ItemsViewScrolledEventArgs in CollectionView when IsGrouped is set to true";
+
+	[Test]
+	[Category(UITestCategories.CollectionView)]
+	public void VerifyGroupedCollectionViewVisibleItemIndices()
+	{
+		App.WaitForElement("Issue17664ScrollBtn");
+		App.Tap("Issue17664ScrollBtn");
+
+		var resultItem = App.WaitForElement("Issue17664DescriptionLabel").GetText();
+		Assert.That(resultItem, Is.EqualTo("Category C item #2"));
+	}
+}
+#endif