Open Radar

 
Summary:
`UICollectionView` sets all items to an estimated / unexpected height during batch updates if cells have the same width as the collection view. If cells are even 1 pixel less than the collection view's width, animations look correct. Attached is a sample project that demonstrates this bug in flow layout, but the issue occurs in all UICollectionViewLayout subclasses that use preferredAttributes / self sizing cells.


Steps to Reproduce:
1. Open sample project
2. Click to remove the item at index 1

[Observe animation issue]

3. Click again to insert that item back at index 1

[Observe animation issue]

4. Change ViewController.swift:73 from `cell.widthConstraint?.constant = collectionView.bounds.width` to `cell.widthConstraint?.constant = collectionView.bounds.width - 1`.

[This makes each cell 1 pixel less wide than the collection view. The issue will no longer occur if steps 1-3 are repeated.]

Expected Results:
Animations should work correctly, even if a cell's width == the collection view's width.

Actual Results:
Animations are broken.

Version/Build:
iOS 9, 10, 11, and 12


Sample project: https://drive.google.com/file/d/1Nay8jkGXwwmJUQKUE-FcQWGqgtW2PQNp/view?usp=sharing