Yup. If you don't do anything for like 10 or 15 minutes, it automatically logs you out. The devs have to balance this issue between two groups: (1) those that use Roundcube in public places and get up and leave, and (2) those that don't ever want to be logged out. The issue is that it becomes a security concern if you don't have this "activity" check in there.
I know you can disable it, but I'm not sure exactly how to do it. You'd have to remove the check from the code, not sure how intensive it'd be to do that though.