Windowed Bug?

Frequently, while playing, the game randomly goes from full screen to windowed mode. While in this mode, my left click pings the map, none of my abilities work, and I press enter the screen alternates back to full screen then back again to windowed mode. I cannot get out of this until I am able to press escape and restore the game to full screen, and that isn't something I can do until I can--as in pressing escape does no good (to bring up the options page) until the game decides it wants to accept it. My keys are only operational in the chat box during this issue.

What do I do for this?