Open Radar

 
Summary:
If you have a UIStackView that you use to hide and unhide views with animated, and the user toggles it too quickly, the view will get stuck hidden. This happens particularly when you are hiding and unhiding 2 views opposite each other, like when picking the start and end dates for an event in the calendar.

Steps to Reproduce:
See the sample project.

1. Create a UIStackView with 2 views.
2. Have a button for each view, one that hides the first view and toggles the second and the other that does the opposite.
3. Mash on the first button quicker than the animation.
4. Tap the second button.

Expected Results:
Everything should end up eventually, with the hidden values of the last ones set.

Actual Results:
One or both of the view’s hidden properties will be stuck to true. Even outside of an animation block, long after the animations have finished, if you set hidden to false and turn around and check the value, it will be true. Mashing some more eventually gets things unstuck.