User mentions in gist comments have no typeahead/autocomplete

Expected Behavior:
When adding comments in most places in Github, you can use @username to mention a user and as you type a typeahead/autocomplete will display as you type. I’d expect this to be the same in gist comments, and I could’ve sworn this worked before but could be wrong.

Behavior:
No typeahead appears when using @username in gist comments. I have to copy the name. There is even the @ symbol in the text tools as it exists everywhere else but it just doesn’t open a typeahead when used in gist comments. It’s not a huge issue but it makes it seem as though the mentions will not notify the users which I believe I saw was a fixed issue elsewhere.