i think i figured out the problem, maybe:
so if i do it using the trackpad on my laptop i am occasionally able to do the job. but i have to move my finger on it in a very particular way, something like in one big motion (although often my trackpad seems too small to actually do a big enough swipe).
with the mouse what appears to be happening is that the offset is "reset" in some way every frame, so the legs never actually move to the side--instead, they jitter in this weird way moving from the center out towards the direction in which i'm moving my mouse. so i can't actually manage to push them all the way to the side, even with the sensitivity turned up quite a bit. (like, i will physically move my mouse an entire foot and make no progress).
one factor that may or may not be compounding this is i know this computer sometimes runs games at up to 240 fps which could be maybe making things worse?