Ah, I feel silly now, it looks like I caused this. At some point I must have added an adblock rule to block the subscribe dialog on fermatslibrary.com, but somehow missed the disable scrolling part.

I just clicked the link again and was able to scroll for a few seconds before it stopped. I toggled ad-block and the subscribe box became visible.

Earlier, I opened the page in a new tab and didn't get to it for a while, so when I switched over to read it, the subscribe box had already disabled scrolling.