Suggestion list for tagging user isn't auto-scrolled when list is long and user presses DOWN key

Description

Steps to reproduce:
1. Create more than 10 users with a name that starts with 'a' character.
2. User views a page in Edit mode.
3. Show the comments pane.
4. Add new comment and type @ +a.
5. Press the DOWN key to scroll to end of suggestion list.

Expected:
The suggestion list should be auto-scrolled, so the user can see which user is currently focused in suggestion list.

Actual:
The suggestion list is not auto-scrolled, so the user cannot see which user is currently focused.