move to core
I believe this plugin should be part of the core set plugins.
Some kind of special autocomplete must first be implemented. It would have to support multiple usernames in one textarea.
@juho-jaakkola indeed. similar functionality is already part of this function https://github.com/ColdTrick/thewire_tools/
if you move this to core, an extraction of the function that shows/hides the popup box would be useful, to allow easy re-use of that method for other popups - to keep the theme consistent.
Very much agree on Move to core!
+1 for putting functionality in core.
This has been done