I'm using the technique outlined here to make a child div 100% width of the browser when the parent is not: https://stackoverflow.com/a/24895631/1184180
This works more or less exactly how you'd want it to, except the implementation of the spec seems to be to add the scrollbar width to the viewport width. So when there is vertical scrolling, the child element is at 100vw + scrollbar width.
This in turn causes the dreaded Superfluous Horizontal Scrollbar to rear its head, which is something nobody wants, ever.
Given that it is due to the behaviour of the browser it seems unlikely there's much that can be done about this - however, I'm wondering if anyone has come up with a genius solution?
The best I can see thus far is setting the width to ~98vw and putting up with the child div background not quite being 100% in width. This does slightly offend my sensibilities, though :/