Ah, I suppose it was just that the window borders were off the screen! Moving the window position through code verified that. Thank you Kimimaru.
GeonBit, I do agree with you on this:
but as I said, I'm just writing a small application for a friend. His monitor has a slightly higher resolution than mine, and I just wanted to be sure monogame wasn't trying to force something I didn't want by forcing the game fullscreen.
Anyways, thank you both for your time and help!