After 2016 Week 4, we've added an autocomplete feature to make it easier to type usernames and project hashtags in comments. It looks like this:
To summon the autocomplete, type a @ (for a username) or a # (for a project name) and then type some text. The autocomplete UI should pop up to help you find the user or project you're looking for if you aren't sure how to spell their username or the project's hashtag.
To select an option, click it or highlight it using the cursor keys and then press Tab or Return.
For more details or to report issues, see T10163: Remarkup Autocomplete Errata.