On top of that (pun intended) you might have trouble with overlays. Each space bar is one “screen”, but that does not factor in any fixed position elements on the page that take up space.
E.g. like right now with the composer up, if I go back to the main window and press space, it’ll still scroll one “screen” which is the vertical height of the browser viewport – but right now the editor and the fixed position topbar are taking up a part of that space.
Wouldn’t this same problem exist on any website that has fixed positon overlays?