What problem does this address?

To improve collaboration and make Notes more actionable, it would be useful to support @mentions inside Notes. When a user is mentioned, they should receive a notification that includes the context of the Note and a link back to the post. This mirrors common patterns in tools like Google Docs, GitHub, and Slack, where @mentions help direct feedback to the right person.

This issue focuses specifically on enabling @mentions and triggering appropriate notifications. It is separate from the rich text work in #73413, though it may build on some of the same UI and input enhancements.

What is your proposed solution?

The feature should allow users to type “@” and search for valid users they can mention, ideally with an inline autocomplete interface. When a Note containing a mention is submitted, the mentioned user should receive an email notification (or future-pluggable notification) that includes the Note content, the post title, and a link that opens the editor to the relevant Note. The implementation will need to respect permissions, ensuring that only mentionable users appear in the autocomplete results as well as handling error messaging when users are not available to mention / don't exist. Care should also be taken to ensure that mentions work consistently in both the floating Notes view and the sidebar Notes panel (archive view). As notifications expand in future iterations, @mentions should integrate cleanly into whatever notification architecture is established. Sanitization and formatting rules should be aligned with rich text support, but the mention system itself should be lightweight, accessible, and fully keyboard navigable.

Open questions:

1. Which user roles should be available for mention autocomplete?
2. Should mentions work only for users with access to the post, or site-wide?
3. How should mentions be formatted visually in Notes (highlight, chip, link, etc.)?
4. How should mentions appear in email notifications?
5. Should mentioned users automatically become “followers” of that Note thread?