Steps to reproduce:

1. Visit e.g. https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html or https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html on an iPhone in Safari.
2. Press the screen and drag left or right.

Expected result:

No left-right scrolling happens.

Actual result:

Browser applies the "overscroll" effect horizontally, letting the scroll go past the edge of the screen, then bounce back.

This is unpleasant for scrolling long docs. It means that an up-down scroll that is not perfectly vertical can get turned into a horizontal scroll.

According to https://stackoverflow.com/questions/15879710/disabling-horizontal-scroll-on-an-iphone-website it sounds like this happens when some element has horizontal overflow. Worth noting that some shorter pages like https://doc.rust-lang.org/nightly/std/string/trait.ToString.html don't exhibit the same behavior. So presumably there is indeed some horizontal overflow on the really long pages like Vec and Iterator, and we need to figure out where it is.

Tested on an iPhone 7 running iOS 15.2.