Steps to reproduce:

1. Go to any form with a 'filetypeslist' field. For example https://qa.moodledemo.net/course/modedit.php?update=5&return=1, 'Accepted file types' in the 'Submission types' section.
2. Click on Choose
3. Click on Cancel
4. Click on Choose
5. Click the first 'Expand' link.

Expected result: that second of choices expands.

Acutal results: nothing happens (well, acutally, in the background, the page scrolls to the top. This is an <a href="#"> with a JavaScript event handler, and it seems that after the dialogue has been closed once, the event handlers stop working.