The horizontal scrollbar is at the very bottom of the page and not shown until you scroll all the way down.

Steps to reproduce:

1. Visit a page with wide content, e.g. https://meta.wikimedia.org/wiki/Wikipedia_20/Material
2. Resize the window so that the page content doesn't fit horizontally.

Actual result:
Horizontal scroll bar is at the very bottom of the page.

Expected result:
Horizontal scroll bar is at the bottom of the window.

Additional information:
This works as expected with the old GUI. Compare https://meta.wikimedia.org/wiki/Wikipedia_20/Material?useskinversion=1 and https://meta.wikimedia.org/wiki/Wikipedia_20/Material?useskinversion=2.

Browser: Firefox 82.0
OS: Ubuntu 18.04.5