When joining alt-tabbed in fullscreen windowed mode, the game will not allow you to use the mouse to look around until you open and close the main menu.
Steps:
Set display mode to fullscreen windowed
Connect to a game
Quickly alt-tab
Get back into the game (you won’t be able to move your mouse now)
This has been around so long, both in solo and online, that I do it out of habit. I’m not sure it is the behaviour of the game, or Windows, though, since I have experienced it in other games.
At least we have a way to re-enable the cursor that takes less that a second.