POC: Add @@ autocomplete for special user types (maintainers/admins)

Summary

This is a proof of concept that demonstrates how to add a new @@ autocomplete trigger that filters users by special types (e.g., maintainers, admins, or other access levels).

How it works

Current @ mention behavior:

• @ triggers autocomplete for all users in the project/group context
• Returns up to 10 users based on search query
• Endpoint: /-/autocomplete/users.json

New @@ mention behavior:

• @@ triggers autocomplete for special users only (maintainers and above)
• Filters users by minimum access level (configurable)
• Endpoint: /-/autocomplete/special_users.json
• Visual indicator (badge) shows these are special users

Changes

Backend (app/controllers/autocomplete_controller.rb)

• Added special_users action that filters users by access level
• Added filter_special_users method to check project/group membership
• Filters for maintainer-level access and above

Frontend (app/assets/javascripts/gfm_auto_complete.js)

• Added @@ trigger configuration
• Added setupSpecialMembers method
• Added visual badge to indicate maintainer/admin status
• Configured backend filtering for the new trigger

Routes (config/routes.rb)

• Added GET /-/autocomplete/special_users endpoint

Demo

When typing @@ in a comment or description field:

1. Autocomplete dropdown appears
2. Only shows users with maintainer+ access
3. Each user has a "Maintainer+" badge
4. Selecting a user inserts @@username

Future enhancements

This POC could be extended to:

• Support different user filters (e.g., @@admins, @@reviewers, @@owners)
• Make access level configurable via query params
• Add different visual indicators for different roles
• Support team-specific mentions (e.g., @@backend-team)

Testing

To test locally:

1. Navigate to any issue/MR comment field
2. Type @@ followed by a search term
3. Verify only maintainers/admins appear in dropdown
4. Verify badge appears next to each user

cc: @gitlab-org/maintainers