Use the arrow keys to move. The way you do this is you have a REALLY big costume on a sprite, switch to it before moving, then switch back to the costume you want to display. (The cube in this example is ~3,500 pixels wide and tall, but you could probably make it bigger to go even further offscreen.) This works by exploiting the fact that Scratch only checks if a sprite is in-bounds when it is moved, not when the costume is switched.
I figured this out on my own, but idk if anyone has found it before, so no credit needed.
