Presumably the implementation is smarter than being defeated by this easy trick, but I too wonder how it works. > Finally, an extra zoom was applied to the vie

Presumably the implementation is smarter than being defeated by this easy trick, but I too wonder how it works. > Finally, an extra zoom was applied to the viewport in fullscreen and maximized modes to use as much of the screen as possible and minimize the size of the empty margins. In that case, the window had a "letterbox" (margins at top and bottom only) or "pillbox" (margins at left and right only) appearance. window.devicePixelRatio was always spoofed to 1.0 even when device pixels != CSS pixels. So presumably the window size is not being reset to real size - firefox just does a smart zoomin. In other words the fake size remains throughout entire session.