Thanks, that was what I was hoping to test. Because of the pixel art elements, the game can't be arbitrarily scaled, only integer scaled (to clean multiples of the base resolution).
Since the base resolution is 640x360; it looks like with the window border, it only scales up once to 1280x720 on your screen - because since the window header is in the way, it can't fill the full 1920x1080. The thing I wanted to test was whether removing that and setting the window to actual fullscreen would make it take up the full 1920x1080 screen or keep the border.