We've had a request from a Codebreaker asking for a way to switch between fullscreen/restored in-game. While it was possible to switch screen mode with the standard Windows shortcut keys (Alt + Enter), there's nothing wrong with making life easier for the players - as long as it's not about simplifying the actual challenges of course.
So, we just released an update to the game that adds buttons to switch between fullscreen/windowed mode.
Thanks for the feedback Ranamo boy!