Unable to @mention Customer or Request Participant

Summary

When adding a comment to a JIRA Service Desk request, the @mention feature does not work for customers or request participants unless those users have permission to browse the project (basically being considered agents) or if they have made a comment on the issue.

Steps to Reproduce

1. Install Service Desk
2. Have a customer create a request
3. As an administrator, view the issue in the Issue Navigator
4. Type a comment and use the @mention feature to mention the customer

Desired Results

• A list of suggestions will appear which you can choose the customer or all the request participant, regardless if they have made a comment or not.

Actual Results

• A list of users appears but this list does not contain the customer or any request participants unless they have made a comment.
  • Agents and collaborators do appear in the list. Any customers and request participants who are agents or collaborators will appear.

Workaround

 Please note that this functionality is available in Jira Service Management Data Center version 4.18+

• Add them manually using markup and Service Desk will notify them as if you had used the normal @mention method:

  [~username]
  

• Have the participant make a comment, and they will be mention-able after that.