I copied it from a previous site that had some weird scrolling requirements, so the top HTML element had width: 100%; height: 100%; overflow-y: hidden; on it, while the body element had overflow-x: hidden; width: 100%; height: 100%; on it. Getting rid of that fixed the issues (unsurprisingly!).
I can't imagine many other sites made this mistake!
The reason I ask is so that I can be more helpful when the issue comes up in other sites