The CheckUser interface currently surfaces user agents when listing users or edits, but there's no way to search by user agent. It would be nice if you could click on a listed user agent and it would then show you all users or edits performed by that user agent.
This may be an expensive query, so we may have to introduce a new database column with hashed user agents that could be used as an index. This would preclude us from being able to do wildcard or prefix searches, however, (for example with a text field) but in almost all cases, the user will want to search for a specific user agent anyway, so I think that's a decent trade-off.
This will require a bit of UX design in consultation with CheckUser users. Things to figure out include:
* When viewing a user agent search, when do we show matching users and when do we show matching edits?
* Do we let users switch between users and edits?
* How does the user get back to the initial CheckUser interface?