Ask HN: What happened to browser scrolling via PgDn?

I do most of my surfing on a desktop, and for navigating around I prefer to use the keyboard.

Lately, the PageDown key isn't working any more on many pages. I press it once, I go down about half a page, once more, maybe a second page, about the third time I end up back at the top. Long pages don't scroll down any more.

This is happening to me in Firefox 64.0 under Windows 7.

That this long-traditional method doesn't work any more is badly frustrating. Can anybody please explain what's going on and what I could do about it?