When typing @mentions, there's an inconsistency in how names are displayed.
The suggestion dropdown shows both username and full name, and the final posted message displays the full name, but the input area only shows the username while typing.

This creates confusion, especially in international teams where usernames may not reflect actual names. Users can't easily confirm who they're mentioning until after the message is posted.

There should be a way to display full names consistently in the input area while typing, matching the behavior of the suggestion list and posted messages. This would improve user experience and reduce mention mistakes.

Similar functionality exists in other chat tools like Slack, where full names are displayed during input.

Related GitHub issue: https://github.com/mattermost/mattermost/issues/32143