@user search result in Comment field limited to 5 results only
When responding to a comment, @user search result is limited to only 5 people when there could be a lot more. Michael is a very common contributor name and you may never find the right person in the drop-down list.
Workaround appears to be searching by last name, and you have better luck in finding the person you are looking for.
The list of results will need to remain limited, because we might have too many matches. GitHub here also seems to only show 5 matches.
Looking at this example, we should improve sorting:

We should rethink the back-end for this. At the moment, on pontoon.mozilla.org we're fetching 780 kB compresed / 2.85MB raw data from /get-users/ during/after the translate view's initial load, and then only using it for @-completion when adding a comment. This is... sub-optimal.
As an alternative, we could/should add query parameters to the API endpoint and only call it when actively doing @-completion, which would allow us to filter and prioritise the list according to the current locale, project and even entity. This might add a negligible delay to the dialog when editing, but would also cut 35% from Pontoon's transferred filesize.