As long as the device doesn't move enough that the original center of mass is no longer inside the device, then externally reactionless motion can be obtained b

As long as the device doesn't move enough that the original center of mass is no longer inside the device, then externally reactionless motion can be obtained by simply shifting mass inside. Other behavior thought impossible like creation of positive/negative mass pairs or teleportation of internal mass would allow to extend the same mechanism to arbitrary movement.