[iOS] Fix CollectionView ScrollOffset not resetting when ItemsSource changes

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

@@ -0,0 +1,7 @@
+namespace Microsoft.Maui.Controls.Handlers.Items;
+
+// Implemented by delegators to allow explicit scroll-tracking reset when ItemsSource changes.
+internal interface IScrollTrackingDelegator
+{
+	void ResetScrollTracking();
+}

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

@@ -414,6 +414,15 @@ public virtual void UpdateItemsSource()
 			CollectionView.ReloadData();
 			CollectionView.CollectionViewLayout.InvalidateLayout();
 
+			// iOS/MacCatalyst: UIKit does not reset ContentOffset during ReloadData.
+			// ResetScrollTracking must run before the assignment so the UIKit-triggered
+			// scrollViewDidScroll callback computes delta from zero, not the stale previous offset.
+			if (CollectionView.ContentOffset != CoreGraphics.CGPoint.Empty)
+			{
+				(Delegator as IScrollTrackingDelegator)?.ResetScrollTracking();
+				CollectionView.ContentOffset = CoreGraphics.CGPoint.Empty;
+			}
+
 			(ItemsView as IView)?.InvalidateMeasure();
 		}
 

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

@@ -8,7 +8,7 @@
 
 namespace Microsoft.Maui.Controls.Handlers.Items
 {
-	public class ItemsViewDelegator<TItemsView, TViewController> : UICollectionViewDelegateFlowLayout
+	public class ItemsViewDelegator<TItemsView, TViewController> : UICollectionViewDelegateFlowLayout, IScrollTrackingDelegator
 		where TItemsView : ItemsView
 		where TViewController : ItemsViewController<TItemsView>
 	{
@@ -25,6 +25,12 @@ public ItemsViewDelegator(ItemsViewLayout itemsViewLayout, TViewController items
 			_viewController = new(itemsViewController);
 		}
 
+		void IScrollTrackingDelegator.ResetScrollTracking()
+		{
+			PreviousHorizontalOffset = 0;
+			PreviousVerticalOffset = 0;
+		}
+
 		public override void Scrolled(UIScrollView scrollView)
 		{
 			var (visibleItems, firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex) = GetVisibleItemsIndex();

src/Controls/src/Core/Handlers/Items2/iOS/ItemsViewController2.cs

@@ -6,6 +6,7 @@
 using System.Linq;
 using CoreGraphics;
 using Foundation;
+using Microsoft.Maui.Controls.Handlers.Items;
 using Microsoft.Maui.Controls.Internals;
 using Microsoft.Maui.Graphics;
 using PassKit;
@@ -296,6 +297,15 @@ public virtual void UpdateItemsSource()
 			ReloadData();
 			CollectionView.CollectionViewLayout.InvalidateLayout();
 
+			// iOS/MacCatalyst: UIKit does not reset ContentOffset during ReloadData.
+			// ResetScrollTracking must run before the assignment so the UIKit-triggered
+			// scrollViewDidScroll callback computes delta from zero, not the stale previous offset.
+			if (CollectionView.ContentOffset != CoreGraphics.CGPoint.Empty)
+			{
+				(Delegator as IScrollTrackingDelegator)?.ResetScrollTracking();
+				CollectionView.ContentOffset = CoreGraphics.CGPoint.Empty;
+			}
+
 			(ItemsView as IView)?.InvalidateMeasure();
 		}
 

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

@@ -9,7 +9,7 @@
 
 namespace Microsoft.Maui.Controls.Handlers.Items2
 {
-	public class ItemsViewDelegator2<TItemsView, TViewController> : UICollectionViewDelegateFlowLayout
+	public class ItemsViewDelegator2<TItemsView, TViewController> : UICollectionViewDelegateFlowLayout, IScrollTrackingDelegator
 		where TItemsView : ItemsView
 		where TViewController : ItemsViewController2<TItemsView>
 	{
@@ -26,6 +26,12 @@ public ItemsViewDelegator2(UICollectionViewLayout itemsViewLayout, TViewControll
 			_viewController = new(ItemsViewController2);
 		}
 
+		void IScrollTrackingDelegator.ResetScrollTracking()
+		{
+			PreviousHorizontalOffset = 0;
+			PreviousVerticalOffset = 0;
+		}
+
 		public override void Scrolled(UIScrollView scrollView)
 		{
 			var (visibleItems, firstVisibleItemIndex, centerItemIndex, lastVisibleItemIndex) = GetVisibleItemsIndex();

src/Controls/tests/TestCases.HostApp/Issues/XFIssue/Issue7993.xaml

@@ -12,12 +12,13 @@
         </Grid.RowDefinitions>
 
         <StackLayout Grid.Row="0" Orientation="Vertical" Spacing="5" BackgroundColor="Beige">
-            <Label LineBreakMode="WordWrap" Text="Scroll down into the list to increase vertical offset. Click NewItemsSource to reset items source. Verify that vertical offset becomes zero." HorizontalTextAlignment="Center" VerticalTextAlignment="Center"/>
-            <Label x:Name="Label1" Text="VerticalOffset: 0" HorizontalTextAlignment="Center"/>
-            <Button Text="NewItemsSource" Clicked="NewItemsSourceClicked" HorizontalOptions="Center"/>
+            <Label LineBreakMode="WordWrap" Text="Tap ScrollToEnd to scroll the list, then tap NewItemsSource. Verify that vertical offset becomes zero." HorizontalTextAlignment="Center" VerticalTextAlignment="Center"/>
+            <Label x:Name="Label1" AutomationId="VerticalOffsetLabel" Text="VerticalOffset: 0" HorizontalTextAlignment="Center"/>
+            <Button Text="ScrollToEnd" AutomationId="ScrollToEnd" Clicked="ScrollToEndClicked" HorizontalOptions="Center"/>
+            <Button Text="NewItemsSource" AutomationId="NewItemsSource" Clicked="NewItemsSourceClicked" HorizontalOptions="Center"/>
         </StackLayout>
 
-        <CollectionView Grid.Row="1" AutomationId="CollectionView7993" ItemsSource="{Binding Items}" Scrolled="CollectionView_OnScrolled">
+        <CollectionView x:Name="CollectionView7993" Grid.Row="1" AutomationId="CollectionView7993" ItemsSource="{Binding Items}" Scrolled="CollectionView_OnScrolled">
             <CollectionView.ItemsLayout>
                 <LinearItemsLayout Orientation="Vertical" ItemSpacing="5"/>
             </CollectionView.ItemsLayout>

src/Controls/tests/TestCases.HostApp/Issues/XFIssue/Issue7993.xaml.cs

@@ -17,6 +17,15 @@ void CollectionView_OnScrolled(object sender, ItemsViewScrolledEventArgs e)
 		Label1.Text = "VerticalOffset: " + e.VerticalOffset;
 	}
 
+	void ScrollToEndClicked(object sender, EventArgs e)
+	{
+		var vm = BindingContext as ViewModel7993;
+		if (vm?.Items.Count > 0)
+		{
+			CollectionView7993.ScrollTo(vm.Items[vm.Items.Count - 1]);
+		}
+	}
+
 	void NewItemsSourceClicked(object sender, EventArgs e)
 	{
 		BindingContext = new ViewModel7993();

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/XFIssue/Issue7993.cs

@@ -1,5 +1,4 @@
-#if TEST_FAILS_ON_CATALYST && TEST_FAILS_ON_IOS //In MacCatalyst, the DragCoordinates is not supported. On the iOS platform, scroll position is not reset while update the itemsource. Issue: https://github.com/dotnet/maui/issues/26366
-using NUnit.Framework;
+using NUnit.Framework;
 using UITest.Appium;
 using UITest.Core;
 
@@ -17,16 +16,12 @@ public Issue7993(TestDevice testDevice) : base(testDevice)
 	[Category(UITestCategories.CollectionView)]
 	public void CollectionViewVerticalOffset()
 	{
-		var colView = App.WaitForElement("CollectionView7993");
-
+		App.WaitForElement("CollectionView7993");
 		App.WaitForElement("VerticalOffset: 0");
-		App.DragCoordinates(colView.GetRect().Width - 10,
-			colView.GetRect().Y + colView.GetRect().Height - 50,
-			colView.GetRect().Width - 10,
-			colView.GetRect().Y + 5);
+		App.Tap("ScrollToEnd");
 		App.WaitForElement("19");
+		App.WaitForNoElement("VerticalOffset: 0");
 		App.Tap("NewItemsSource");
 		App.WaitForElement("VerticalOffset: 0");
 	}
-}
-#endif
+}