I've had this problem a few times, and i think it could possibly be addressed. Essentially, when the game starts and you are in the class-pick screen, there is instead the scoreboard. You can't exit, can't see your mouse, can't pick, you have to close the game out. It is a known issue, but it was supposed to be fixed. Same problem, hitting tab before class selection screen (i just tap my keys when bored).
Hold the scoreboard screen and you get the class selection screen as long as you hold the key Once you've picked your class, click Close then the scoreboard behaves normally
