I think this just cropped up, because I'm pretty sure it's something I've done in the past.
Type "@" and begin entering a user name. Hit enter upon selecting the user you want to mention. Then, hit enter again.
The username gets cleared, and you're left with a basic @ symbol again. You have to type garbage text after a username in order to hit a return on the same line.
If you delete the garbage text, and then hit enter, it clears the username again.
Win8.1 Pro, Chrome v40.0.2214.93 m
// PSN: Elvenshae // WotC: Elvenshae
The Disappearance of Inigo Sharpe: Tomas à Dunsanin