There is no username popup suggesting completion when typing an editor's user name prefix.
Steps to reproduce:
- Find a post by AUser that was edited by UserB
- Write a comment addressing
When having typed
@U, the completion popup should suggest
No popup, as if there were no such user in the list of addressable users.
Notifications apply to the author, commenters, and editors of the question or answer that you are commenting on—users not in that list cannot be notified. You can also notify a ♦ moderator who closed the question.
The popup can easily be seen as an indicator whether the user whose name I'm typing will receive my message, since, while typing a user's name and the name disappears from the suggestion list, it means I mistyped and the user will not be notified.
This behavior suggests that editors will not be notified at all, while they actually are. This is why I classified this as a bug.