Open Radar

 
Summary: During the completion block of animating content within a row in a collection view, if you reload rows in the collection view to have a larger height, and the user scrolls the UICollectionView rapidly at the same time, a "phantom" row can be created in the collection view.  This row is an additional, visible row which seems to have been forgotten by the inner workings of the collection view. 

Note: I have seen these rows appear without scrolling and in other scenarios, but this seems to be the most consistent way to reproduce the problem.  I have also seen this happen in the UITableView but it is far harder to reproduce.

Steps to Reproduce: See the attached project.  It has an example of code that can cause this crash.  The steps when running the project are:

1. Click reload on 3-6 cells.
2. Scroll the collection view up and down rapidly

Expected Results: All of the cells reload correctly.

Actual Results: The cells reload correctly, but there are one or more additional rows created

Regression: I am not aware of any version of iOS where this works.

Notes: See the attached project for an example.