Moving windowed game to (larger) second monitor and then going fullscreen broke game

[I'm proud of you, son]

Fullscreen will force the game to expand ONLY on my primary monitor, but it is using the dimensions of my second monitor. So now full screen 1600*900 on my laptop (primary monitor) is shrunk. It will not go back, I have to play in windowed mode.

Win 10 64-bit, Home edition.