- Detection of when auto-complete should pop up
- Determining the list of items that can be auto-completed and displaying them
- Finishing auto-complete (whether it be because it's no longer applicable, or an item was inserted)
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There's an event handler that fires for key and click events, and checks whether the cursor is currently on a word that's preceded by an "@". If so, there's your first bullet point; if not, there's the third (note that even when an item was inserted, the cursor is still on the word, and the auto-completer is still there; otherwise you couldn't tab-cycle through the several users all starting with the same letter(s) you typed).