Content can overlap open menu in mobile Safari

[vwkd]

Opening the menu in mobile Safari allows the main content to be scrolled around. It seems the overflow is not being hidden. I didn't test on other mobile browsers.

muvl31.mp4

Safari on iOS 15.0.1

Reference: rust-lang/book#2905

[ehuss]

This should now be fixed via #1994 in 0.4.26.