T232601 implemented the fundamental components that make in-surface autocomplete [for @ mentioning] possible.
This task is about building off that work to make the username completion list  work in the following ways described in "Expected behavior."
When someone types @, they will see...
- The username completion list, populated with the usernames of the people who have already commented anywhere on the talk page they are viewing.
- The usernames in the completion list will be sorted alphabetically.
- The usernames shown in the username completion list will appear in the username results list exactly as they appear on the page. Meaning, if someone has a custom signature such that the username that is appended to the comments they post on talk pages  is different from the username that appears in the link to their user page , their alias will show up in the username results list.
When someone selects an alias from the username completion list, they can expect the following to happen:
- The Reply text input will show @ + alias (read: the same as what is shown in the username completion list).
When someone publishes a comment containing a ping to someone with an alias set, they can expect the following to happen:
- A ping will be sent in the same way it would be to a person who does not have an alias set.
- For people who have aliases set, is it customary for people wanting to @-mention them to refer to them by alias or by their registered username?
- What if a custom signature and a real username collide?
- What if multiple people on a page have the same alias?
- Will it confuse the user if they pick @alias in the dropdown and what's output in the wikitext is [[User:RealName]]?
- Do we indicate alias-status in the completion dropdown?
- Will it confuse the user that completion of some names only works on some pages for inscrutable reasons?
- Do we assume everything that links to a user-page is intended as an alias?
- The "Open questions" have been answered
- The functionality described in "Expected behavior" has been implemented