I know how to search a user by name. But I want to search for tags and a particular username. Searching for the userid does not work, because I do know that there are several userids for the same username (and actual person). So this is different to the other questions posed here already.
The reason for it being that some users (for rather technical reasons) get a new userid from time to time. Actually, even I got a new one for no reason (that I was able to understand at that time). As an example I want to search
In the below discussion it was suggested to add a user merge request for user:mat. Which I subsequently did. Unfortunately, this was not really welcomed - implying that I want to force a user to merge who supposedly does not want to. In any case this suggests that searching for the username is a missing functionality.