I noticed that expander has two events: Expanding and Collapsed that are named differently.

Expanding implies the event is fired before the control is expanded and Collapsed implies after. Therefore, a difference in naming here concerned me so I went to check the code. Sure enough, there seems to be a mismatch. According to the code below all events are fired BEFORE the is expanded state is applied meaning both events should be named with a 'ing' suffix. Is my understanding correct here? If so this is actually a bug.

The differences in naming is actually quite important as in the future four events should be added to cancel expanding/collapsing. #3279 (comment). There should be four total events in the end:

1. Expanding <- Can cancel
2. Expanded
3. Collapsing <- Can cancel
4. Collapsed