So obviously when you enter fullscreen the game takes up the entire screen, but it could be useful to be able to override the dimensions so that the game only takes up a smaller section with the rest being black borders or whatever. For example, if your fullscreen is 1980x1080 but you specify an override of 1800x1080, then there would be 180 pixels of black border on the sides. Two benefits of this I can think of would be:
- On Macs with notches, when you enable fullscreen it extends into the notch area for some reason, which cuts off the top (where the progress bar is) and offsets button hitboxes. But if you shrink the top border by however many pixels the notch is then the issue would be fixed.
- If someone has a monitor with unusually big/shaped dimensions, then sometimes levels look ugly because the elements don't extend to where the border is (since the creator wasn't anticipating someone with that wide of a screen playing it). So they may want to limit their screen to a smaller width, but still keep the benefits of fullscreen over windowed like having no taskbar.