Author here. That's actually great feedback. I accidentally broke the container scroll with a single line CSS change to fix something else, ugh. Should be fixed now.
Works now! Another free suggestion: when you drag a bit fast, since the animation is a bit slow, sometimes the boundary between before/after will barely move before your cursor makes it to the edge and reset to the middle, which is a bit jarring / doesn't let me really see anything. Should either make the animation faster, or put the reset threshold outside of the container somehow.