As I recall, the mouse sets acceleration, but not the position of the gamer's piece, so it is possible to have movement even if the mouse is left alone. FWIW,

As I recall, the mouse sets acceleration, but not the position of the gamer's piece, so it is possible to have movement even if the mouse is left alone. FWIW, I just tried the emulator on MacOS Sierra and Chrome and it works just like I remember it.