Suggestion list not refreshed if user removes all characters following '@'

Description

Steps to reproduce:

1. Views a page in Edit mode.
2. Show the comments pane.
3. Add new comments and type @ + a character that occurs in some user names.
Note: Suggestion list appears with matched users.
5. Press the BACKSPACE key to remove all characters following '@'.

Expected:
The suggestion list should be refreshed. If there is no character following '@', no user should listed, and text "No matching users" should appear.

Actual:
Users who match the previous filter are still displayed.