Mentions change what Enter and Tab do right after `@word` and open a
list for everyone, so they get a setting next to the empty subject
warning (`recipientMentionsEnabled`, on by default, translated in
every locale). Turned off, the composer offers no candidates and the
editor keeps its keys.
While the list is open, the editor now carries `aria-autocomplete`,
`aria-controls` and `aria-activedescendant`, as the To/Cc
autocomplete does, so screen readers hear the list and the highlighted
name. Each row now leads with the `@name` that will be inserted,
followed by the full name and the address: for a surname-first name
such as "Nagy János" the inserted word is not what the name alone
suggests.
Linus Rath
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