Flickering/ALT Tab issue

So randomly throughout my games the window will flicker a few frames and essentially alt tab me from the window. My mouse is kicked out of game and instead goes off screen and screws me up majorly in a team fight. It happens many times throughout every game, maybe every minute or 2. It's an easy fix to get back into the game now since I'm used to it (press ALT + Tab and right click ). I never had this issue patch 4.20 or earlier so it has to be the new 4.21 patch. Not sure if its a common problem.

Any solutions or fixes are appreciated. Otherwise I hope to see this fixed next patch.