While browsing M.SE, I came along this comment.
Out of curiosity I tried to put a comment-reply. Copy-paste is working (this time), but can't we have an autocompletion right after the @?
BTW: Which characters are allowed in user names? I got this, saying that $\LaTeX$ won't be rendered.
Would an @ be allowed? I thought about giving it a try, but I fear that comment replies won't work anymore since only 1 @ seems to be allowed in a comment, see here for some experiments.