[iOS] Fix CollectionView ScrollOffset not resetting when ItemsSource changes (#34488)

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### Issue Details

- On iOS, when using CollectionView with the Scrolled event, the
VerticalOffset value doesn't reset to 0 when changing the ItemsSource.
While the collection view displays the new items correctly, the reported
scroll position remains at the previous offset value instead of
resetting to the top position.

### Root Cause
- On iOS, UICollectionView.ReloadData() does not reset ContentOffset.
ContentOffset retains its previous value, meaning any subsequent
Scrolled callback—which derives VerticalOffset and delta values directly
from ContentOffset—reports the stale, non-zero offset. As a result,
MAUI's VerticalOffset never resets to 0 after the ItemsSource changes.

### Description of Change

- Added logic in ItemsViewController.cs and ItemsViewController2.cs to
reset CollectionView.ContentOffset to zero only when it’s not already at
the origin. This ensures the scroll position resets correctly when the
ItemsSource changes on iOS/MacCatalyst.
- Before resetting the offset, ResetScrollTracking() is invoked via the
internal IScrollTrackingDelegator interface so that the
scrollViewDidScroll callback computes the delta from zero instead of
using a stale offset.

**Test and UI improvements:**

- Updated the test page (Issue7993.xaml) to add a "ScrollToEnd" button
for reliably scrolling the list, and updated label and button
identifiers for improved test automation.
- Added the ScrollToEndClicked handler in Issue7993.xaml.cs to
programmatically scroll to the end of the list, supporting the new test
flow.
- Removed platform-specific test skips and refactored the test
(Issue7993.cs) to use the new "ScrollToEnd" and "NewItemsSource"
buttons, improving reliability and making the test applicable to
iOS/MacCatalyst.


### Issues Fixed
Fixes #26366
Fixes #33500

### Validated the behaviour in the following platforms

- [ ] Windows
- [ ] Android
- [x] iOS
- [x] Mac

### Output
| Before | After |
|----------|----------|
| <video
src="https://github.com/user-attachments/assets/a374859f-25ce-445f-a53f-6354c2d3f201">
| <video
src="https://github.com/user-attachments/assets/2bdc2dd4-43be-41ef-810c-e4098cba791f">
|

Author: SyedAbdulAzeemSF4852

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

@@ -427,6 +427,15 @@ 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

@@ -9,7 +9,7 @@
 
 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>
 	{
@@ -26,6 +26,12 @@ 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;
@@ -298,6 +299,15 @@ 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

@@ -10,7 +10,7 @@
 
 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>
 	{
@@ -27,6 +27,12 @@ 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
+}