When pressing pagedown, sticky header covers top of new page

[nyanpasu64]

Ever since #1070, the sticky header does not disappear when I scroll down.

Every time I press the Page Down keyboard key, there are a few lines of text I could not read before pressing Page Down, and are covered by the floating header after I press Page Down.

This occurs on Firefox 70.0.1 (64-bit). Normally Firefox (not Chrome) has code to detect sticky headers and reduce the distance that PageDown travels. However Firefox fails to detect the header on this page. (I'm not sure why and haven't researched what DOM/CSS structures trigger this issue yet. The issue is on https://bugzilla.mozilla.org/show_bug.cgi?id=1327869)

Example page: https://doc.rust-lang.org/stable/book/ch17-01-what-is-oo.html

[ehuss]

@rnitta can you take a look at this?