screen peeping
Looking at a friends screen while playing a split-screen game with them.
Person 1: "Dude! Stop screen peeping!"
Person 2: "I wasn't screen peeping."
Person 1: "Yeah you were. How else could you know that that was my hiding spot?"
Person 2: "I wasn't screen peeping."
Person 1: "Yeah you were. How else could you know that that was my hiding spot?"