R1:7618a4e056ea

Allow standard page body panel to scroll on overflow-x

Summary:
This seems like the least-bad solution to the issues mentioned in T684: when we
need to x-scroll the main page area, scroll that div rather than the surrounding
page chrome.

I played around with a bunch of other possible solutions but they all seem bad
in some way or another. The tricky part here is that I want the real background
to be grey so that the footer color is grey even if the page is very short and
the browser window is very tall.