Issue: #26798 - [iOS]VerticalOffset Not Reset to Zero After Clearing ItemSource in CollectionView
Related Issue: #21708 - CollectionView.Scrolled event offset isn't correctly reset when items change (Android)
Platforms Affected: iOS, MacCatalyst (labeled platform/ios, collectionview-cv2)
Files Changed: 2 implementation files, 3 test files

Issue Summary

When a CollectionView is scrolled and then ItemsSource is cleared/modified, the VerticalOffset and HorizontalOffset in ItemsViewScrolledEventArgs do not reset to 0. The old offset values are retained even though there are no visible items. Affects iOS 18.0, macOS 15.1. Reported as a regression of Xamarin.Forms behavior.

Fix Approach

The PR removes the early return from Scrolled() when !visibleItems, and instead sets contentOffsetX/Y to 0 when there are no visible items. This ensures a Scrolled event fires with offset = 0 after the items source is cleared.

Modified Files:

• src/Controls/src/Core/Handlers/Items/iOS/ItemsViewDelegator.cs (+2/-5)
• src/Controls/src/Core/Handlers/Items2/iOS/ItemsViewDelegator2.cs (+2/-5)

New Test Files:

• src/Controls/tests/TestCases.HostApp/Issues/Issue21708.xaml (+69)
• src/Controls/tests/TestCases.HostApp/Issues/Issue21708.xaml.cs (+52)
• src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue21708.cs (+23)

Key Findings & Concerns

1. Critical Bug in Fix (Copilot Reviewer Identified)

After removing the early return when !visibleItems, lastVisibleItemIndex is -1 and source.ItemCount can be 0. This causes the RemainingItemsThreshold logic to fire incorrectly:
SendRemainingItemsThresholdReached() fires incorrectly
fires for threshold >= 0

Required fix: Add guard if (!visibleItems) return; before the RemainingItemsThreshold switch.

2. Test Naming Inconsistency

PR says it fixes #26798 (iOS issue) but test files are named Issue21708 (Android issue #21708). The [Issue] attribute references issue 21708. Should reference 26798 or both.

3. Test Assertion Quality

Assert.That(App.FindElement("Label").GetText(), Is.GreaterThan(" string comparison with Is.GreaterThan. Works in practice but semantically wrong.0"))

4. Missing Newline at End of Files

All three new test files are missing newline at end of file.

Reviewer Discussion

Copilot reviewer left 2 inline comments (both flagging the RemainingItemsThreshold regression in Items/ and Items2/).

Fix Candidates

Result PASSED:
Platform: iOS
Mode: Full Verification (verify-tests-fail-without-fix skill)

• Tests FAIL without fix (bug is present, offset not resetting)
• Tests PASS with fix (fix correctly resets offset to 0)

Test: VerifyCollectionViewVerticalOffset in Issue21708
Fix Files Verified: ItemsViewDelegator.cs, ItemsViewDelegator2.cs

Fix Candidates

Cross-pollination: 3 rounds; models exhausted at round 3 (3 of 5 models saying NO NEW IDEAS for all approaches tried).

Exhausted: Yes (for practical purposes; remaining ideas from models are increasingly complex architectural alternatives)

Selected Fix: PR's fix with required guard The PR's 2-line-per-file change is the simplest approach, but it needs a guard if (!visibleItems) return; (or if (lastVisibleItemIndex == -1) return;) before the switch (itemsView.RemainingItemsThreshold) block to prevent SendRemainingItemsThresholdReached() from firing incorrectly when items are cleared. Alternatively, Attempt 5 (Gemini) avoids the regression entirely by keeping the early return and firing a reset event inside the !visibleItems block, but is more complex.added

Root Cause: The Scrolled() method originally returned early when there were no visible items (!visibleItems), preventing any offset reset event from firing when ItemsSource is cleared while scrolled. The previous offsets were retained because no SendScrolled call was made. The fix must fire a Scrolled event with 0 offsets when items become empty.

Regression Risk: The PR's approach of removing the early return exposes the RemainingItemsThreshold switch to lastVisibleItemIndex == -1 and source.ItemCount == 0, causing SendRemainingItemsThresholdReached() to fire incorrectly on any clear of ItemsSource (when threshold != -1). This is a critical regression.

Final Recommendation: REQUEST CHANGES##

Summary

PR #34153 fixes a real iOS/MacCatalyst bug where ItemsViewScrolledEventArgs.VerticalOffset and HorizontalOffset fail to reset to 0 when ItemsSource is cleared while the CollectionView is scrolled. The fix approach is simple and but it introduces a critical regression in the RemainingItemsThreshold logic that must be addressed before merging. The Copilot reviewer already flagged this issue with inline comments.correct

Gate verified: tests FAIL without fix, PASS with fix (iOS, Issue21708).

Root Cause

When ItemsSource is cleared, iOS fires the UIScrollView.Scrolled delegate while IndexPathsForVisibleItems is empty. The original code returned early (if (!visibleItems) return;), so no Scrolled event was sent to the MAUI layer and PreviousHorizontalOffset/PreviousVerticalOffset kept their stale values. When new items were added later, the stale values caused incorrect deltas and offset values.

Fix Quality

The PR's removing the early return and setting offsets to 0 via ternary when ! is correct for fixing the offset bug. It passes the gate tests and is the simplest possible change (2 lines per file). However, it introduces a regression:visibleItems approach

Critical regression (RemainingItemsThreshold):

After removing the early return, lastVisibleItemIndex can be -1 when visibleItems is false, and source.ItemCount can be 0 when items are cleared. The following switch logic then fires incorrectly:

case 0:
 fires incorrectly!
    if (lastVisibleItemIndex == source.ItemCount - 1)
        itemsView.SendRemainingItemsThresholdReached();
    break;
default:
 fires incorrectly!
    if (source.ItemCount - 1 - lastVisibleItemIndex <= itemsView.RemainingItemsThreshold)
        itemsView.SendRemainingItemsThresholdReached();
    break;

Any user using RemainingItemsThreshold != -1 would see RemainingItemsThresholdReached fire every time items are cleared from the collection. This is a high-severity regression.

The Copilot reviewer already identified this in both files (Items/ and Items2/) and suggested adding a guard.

Required Changes

1. Add guard before RemainingItemsThreshold switch (both files)

In src/Controls/src/Core/Handlers/Items/iOS/ItemsViewDelegator.cs and src/Controls/src/Core/Handlers/Items2/iOS/ItemsViewDelegator2.cs, add after itemsView.SendScrolled(...):

if (!visibleItems)
    return;

switch (itemsView.RemainingItemsThreshold)
{ ... }

This matches the pattern suggested by the Copilot reviewer.

2. Test naming consistency (minor)

Test files are named Issue21708 but the PR description says it fixes #26798. Issue #21708 is a related but different (Android) issue. If the intent is to test the iOS behavior, the test should reference #26798 or the test description should be updated to clarify the dual-platform coverage.

3. Missing newlines at end of files (cosmetic)

All three new test files are missing newlines at end of file.

Code Review Findings

RemainingItemsThreshold regression (flagged by Copilot reviewer)

• Files: ItemsViewDelegator.cs, ItemsViewDelegator2.cs (both files)
• Problem: Removing if (!visibleItems) return; exposes the switch (itemsView.RemainingItemsThreshold) to lastVisibleItemIndex == -1 and source.ItemCount == 0, causing SendRemainingItemsThresholdReached() to fire on every ItemsSource clear when threshold != -1
• Recommendation: Add if (!visibleItems) return; immediately after the itemsView.SendScrolled(itemsViewScrolledEventArgs); line in both files

• Test naming: Issue21708 test references the Android issue (CollectionView.Scrolled event offset isn't correctly reset when items change on Android #21708); should reference the iOS issue ([iOS]VerticalOffset Not Reset to Zero After Clearing ItemSource in CollectionView #26798) that this PR actually fixes, or add a comment explaining the dual-platform intent
• Test assertion quality: Is.GreaterThan("0") on line comparing a string label text is a lexicographic string comparison. Works in practice (scroll offset > 0 as string), but Is.Not.EqualTo("0") would be more semantically correct
• Missing EOF newlines: Three new test files are missing newlines at end of file

Looks Good

• The core fix logic is correct and minimal (2 lines changed per file)
• Applied symmetrically to both Items/ (legacy) and Items2/ (current) iOS handlers
  Empty flow correctly
• PR description has the required NOTE block at the top
• PreviousHorizontalOffset/PreviousVerticalOffset correctly reset to 0 in the empty case (via the ternary assignment)

Try-Fix Exploration

7 independent approaches were tested. Notable finding: Approaches that preserve the early return avoid the regression entirely.

Among alternatives, Gemini's Attempt 5 avoids the regression cleanly:

if (!visibleItems)
{
    // Fire reset event only if we were previously scrolled
    if (PreviousHorizontalOffset != 0 || PreviousVerticalOffset != 0)
    {
        // fire Scrolled event with 0 offsets
        PreviousHorizontalOffset = 0;
        PreviousVerticalOffset = 0;
    }
 RemainingItemsThreshold NOT affected
}

However, the simplest fix remains the PR's approach + the guard from the Copilot reviewer.

Title & Description Review

Title: [iOS] Fix VerticalOffset Update When Modifying CollectionView.ItemsSource While Scrolled

• Has platform prefix [iOS]
• Describes the fix clearly
• Suggestion: Could be simplified to [iOS] CollectionView: Reset scroll offsets to zero when ItemsSource is cleared

Description: Has NOTE block, brief description. Missing root cause and technical detail, but acceptable for a small focused fix.

What NOT to Do (for future agents)

Don't remove if (!visibleItems) return; without adding a guard before the RemainingItemsThreshold lastVisibleItemIndex == -1 and ItemCount == 0 cause spurious SendRemainingItemsThresholdReached() callsswitch -
Don't reset Previous offsets alone (Attempt stale contentOffsetX/Y values are still used in event args even after resetting the cached fields4) -