Groups without projects do not autocomplete for mentioning

Summary

Gitlab groups do not autocomplete when mentioning unless the group also has a public project.

Steps to reproduce

• Create a sub-group and add some members.
• Only the sub-group members can mention the group.
• Create a public project under the group.
• Everyone who has access to the group can now mention the sub-group

Example Project

We discovered this when we created sub-groups under: https://gitlab.com/gitlab-com/gitlab-ux.

The sub-groups could only be autocompleted when there was a project created under them. These groups were created only for mentions, so we had to create dummy projects as a workaround.

What is the current bug behavior?

Group does not show up in autocomplete

What is the expected correct behavior?

You should not have to create a project in order for group members to use the sub-group for mentions.

Relevant logs and/or screenshots

Screenshot - no autocomplete for @gitlab-org/graphql-experts

Screenshot - Group still available when typed out explicitly for @gitlab-org/graphql-experts

Output of checks

This bug happens on GitLab.com

Possible fixes

The issue is somewhere around here https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/services/projects/participants_service.rb#L49. Specifically the public_or_visible_to_user(current_user) is suspect, and maybe also the chained visible_groups = project.invited_groups.

Workaround

Create an empty placeholder project within group to allow mentions, see #215070 (comment 480588810)