Open Radar

 
Area:
UIKit

Summary:
UICollectionView has glitchy scrolling issues and cell sizing issues when 
- isPrefetchingEnabled == true
- a UICollectionViewLayout subclass that self-sizes cells is used (but not a UICollectionViewFlowLayout subclass). 

The issue occurs specifically when in the aforementioned scenario, and scrolling up to rubber band / over scroll at the top immediately after the collection view loads. When the user lets go and the over scrolled collection view begins to rubber band back *toward* contentOffset.y == 0 (the final resting state at the top), the contentOffset.y will be briefly set to 0 *during the rubber band animation* - which causes the collection view to jump to offset 0 in the middle of the animation. This is very jarring and noticeable.


Attached is a demo project with a UI test to simulate this over scroll behavior reliably and consistently. See Steps to Reproduce.


Steps to Reproduce:
1. Open sample project. The included project is set to use a simple, self sizing layout by default (subclass of UICollectionViewLayout).
2. Command+U to run the UI test, which will simulate an over scroll at the top in a consistent way each time the test is run.
3. Note the jumpiness. If it's not obvious, re-run the UI test a few more times.

Ways to "fix" the issue:
-  ViewController.swift:24, switch to using flow layout.
or
- ViewController.swift:44, set isPrefetchingEnabled to false.
or
- SelfSizingLayout.swift:26, override _estimatesSizes and return true. The code is already there, so just uncomment it at the top.

Expected Results:
The user should be able to over scroll at the top without jumpy animations. No special internal flags should be needed to achieve correct behavior.

Actual Results:
The over scroll animation at the top is jumpy, due to a mix of prefetching and self sizing cells.

Version/Build:
iOS 11 (and 12)



Demo project: https://drive.google.com/file/d/1nA32eHFbUwKJbFFwyHX1FwqDY0D0ah5V/view?usp=sharing