This sort of thing is the absolute pits. Now that position: sticky is in CSS, are there any good reasons browsers don't rate limit onscroll event handlers?
Holy shit, you're right. It's so bad on mobile. It's broken in so many ways. It scrolls horizontally and vertically. At one point I somehow scrolled the header instead of the body. I want to debug the page just so I avoid the same issues on my own sites.