Upstreamed as https://we.phorge.it/T16126

In an ideal world, typing @ and a letter in a Phab task would not start listing non-disabled user accounts alphabetically, but take context in account (task author, task subscribers, task assignee) and show these usernames as the first proposals if they match the letter(s) entered by the user, followed by the current results.

However, src/applications/people/query/PhabricatorPeopleQuery.php ::getOrderableColumns() only takes 'username' into account, and src/applications/people/typeahead/PhabricatorPeopleDatasource.php ::loadResults() is completely context independent (we have no way to find out which current object our given src/view/form/control/PhabricatorRemarkupControl.php is located in).

PhabricatorCursorPagedPolicyAwareQuery has some options to influence the Vectors but nothing that seems useful for this use case.

That might involve some non-existing query for "anyone involved with an object (task), covering PhabricatorObjectHasSubscriberEdgeType::EDGECONST, ManiphestTask::getOwnerPHID(), ManiphestTask::getAuthorPHID().
And some additional code to inject that before other search results if the entered character(s) match for some of those accounts.

Filing this ticket as having this tracked here is better than getting lost on my local harddisk.