Put your mouse arrow off to the right or left side of the screen while scrolling so the inside of a scrolling signature doesn't screw up the scrolling.

I think the method you've pasted would still allow longer signatures but "break" them in a way that they extend farther than what the human eye could see even if we wanted to scroll, because it would offer no scroll bar.

When I have some time I'll look more into it and see if there's a prettier method but for now just don't let your mouse arrow be on the middle of the screen while scrolling and it won't conflict with the sig scrolling. If you're using a phone, just make sure you keep your finger toward the left side while scrolling so you're not touching sigs. Stop touching my balls.