Comments @ auto-complete doesn't always appear

Question

I've noticed that when I try and add a comment to send to a user using the @ prefix (i.e. @NickCraver) that JavaScript attempts to auto-populate with a little pop-up based on what comments have been made to that question or answer.

However, I've noticed that the pop-up doesn't always appear, even though I'm not mis-spelling the name and that user has posted in the question/answer. There seem to be two possible issues here:

1. The pop-up auto-populates based on other comment posters, the answerer themselves won't appear unless they've also commented. So in this thread - Multiple tags with JavaScript off - If I try and comment to @Shog9, their username doesn't auto-pop up. Or The stackexchange.com 'sites' page doesn't work without JavaScript - @Emmet there doesn't pop-up. This may be either a bug or "by design" or simple oversight.

2. Sometimes, even when a user has posted a comment they don't auto-pop-up. I don't get @ManisHearth against the answer here - The stackexchange.com 'sites' page doesn't work without JavaScript - but s/he has commented. I thought maybe it was because they had posted the answer (see (a)), however @NickCraver does auto-popup for their answer here - "Add" button doesn't work with JavaScript off

Answer 1

Here is official source explaining how this works:

The chat JavaScript has a list of users it knows about, and chooses from them; the Q&A JavaScript just picks the usernames from the displayed comments. In fact, the comment auto-completer could almost 100% be implemented as a user script (and there actually was such a script, and I assume it did pretty much the same thing).

I have bolded the relevant part - as you assumed, it is indeed just taking the list of users who commented on the post. (Post being either a question or an answer)

In July 2011, @lerting the post owner in a comment has been declared as not necessary and removed from the comment, so it was never part of the auto complete list as long as only one user is taking part in the comments.

Now, if user (not the post author) has already commented, while the author will get notification for any further comment you still might want to address your comment specifically to him/her so the @ becomes valid once more even for the post author.

Answer 2

1) Any comment on a post will always notify the poster. You need not explicitly @mention them. This can lead to a bit of annoyance when others hijack the comment thread on your own post, but it's pretty sane otherwise. Comments are for improving the post, so, in a perfect world, each comment would be something that's for the poster to read and possibly incorporate. It's not a perfect world, though--so we do get comments that are not intended for the OP to read. Meh; their fault for hijacking a comment thread.

2.1) Again, the poster is auto-notified. Try it here as well. For some reason, the @reply does autocomplete if the post is a question and the asker has commented, but not for an answer. Inconsistent, but harmless.

2.2) My username is correctly capitalized ManishEarth (the Earth is a random thingy), and I'm a guy. :)

If you don't like any of this, and want to specify who you're talking to, here's a userscript!