I’m using the Rise theme and want to disable the auto-scroll/nudge when a variant is selected" or something like that
What autoscroll you’re talking about?
Generally, it’s a nice idea to provide preview link to the page where the problem can be seen to get better assistance.
Otherwise it’s only a guesswork. Say, page may be scrolled to focus on newly selected variant image.
Or it can be some conflict between theme and app CSS or javascript – hard to tell without looking.