Mentioning a user on a completed task in a PR causes the page to disable input. (BB-15053)

Confirmed in Chrome, firefox, safari, and IE on OSX.

Steps to reproduce:

1. Open a merged PR.
2. Click on the All tasks resolved link in the top right of the page.
3. Click the small chain-link icon to the right of one of the completed tasks.
4. A Comments on prior versions modal will pop up, showing any comments.
5. Click on the Reply link under any comment. (tested & reproduced using both)
6. Type in a @... user reference (in my case @donal), and after a second or 2, the pop up modal will disappear, and the page will become stale. There is nowhere to add input
   since the dialogue box is gone.

Only way to get out of it, is to go back to the original pr.

Screenshare added to internal issue.