Mention autocomplete

The trigger character is the whole trick. A comment box is prose, and prose has no schema, so there is nothing to hang a picker off. Typing @ declares that the next few characters are not prose but a query, and the field switches modes for exactly as long as that query lasts. Choosing a result ends the mode and leaves a token behind: a chip or a link that carries an identity, not the letters someone typed.

That is what separates it from the field-level patterns it looks like. A typeahead or a combobox owns its whole field, and every character in it is part of one query, which is why the widget can be a labelled control with a listbox attached to it. Mention autocomplete lives inside a field whose real purpose is something else, so it has to decide, on each keystroke, whether a query is even in progress. It is the same machinery a slash command uses, with a different sigil and a different catalogue: @ for people, # for issues or channels, / for commands, : for emoji.

The parsing rules are short and every implementation gets them wrong at least once. The trigger only counts at a word boundary, or an email address opens a picker halfway through. The query ends at whitespace, which is the conventional dismissal: typing a space after @ means the reader meant the character literally. Backspacing through the trigger closes the menu. And the token is atomic once inserted, so Backspace removes the whole mention rather than the last letter of a name, and editing around it never leaves half a handle in the text.

Two things then have to survive the send. The stored value is the identity, not the display name, so a person who changes their name does not orphan every mention of them. And the rendered token needs to say what it is to a screen reader (a link with the person’s name is usually enough) and to the reader who has to know that typing a name in prose is not the same as tagging someone. The mention is a notification as much as it is text, which is why an accidental one is worse than a typo, and why the list should be scoped to people who can actually see the thread.