It is nice how typing the @ symbol brings up a list of users already on a post that you're replying to. This makes tagging people quick and easy.
Sometimes the list of users that have replied to a post is greater than the subset sample shown in this dropdown, however. In this case, you can start typing and the results do update, but it seems to return the best match from the Community at large. If I want to tag someone from this particular post I'm replying to, I often have to type out nearly the whole name before it is able to narrow down the search results to the user I want. I would love to be able to start typing just the first two characters of a username and have the list update while prioritizing the users relevant to the post.
Just typing @:
After typing pa:
How many letters I had to type out before finally getting the result for the user who actually made the post I'm replying to: