Steps to reproduce:

1. Recently updated to 4.2.1
2. When viewing any of our maps, the horizontal scrollbar has disappeared

Result:
See screenshot, there is no horizontal scrollbar

Expected:
See screenshot with arrow, there is a horizontal scrollbar

The scrollbar does appear to return (this is how I got the expected screenshot) if you run jQuery(".flickerfreescreen").attr("style", "overflow:auto") in the browser console.