[layout] small larger width for certain viewports

[karlcow]

@magsout Congratulations on the new layout. ㊗️

Around 1227px, there is a slight larger width which means that you can slide horizontally the design.
On the issue page for example https://webcompat.com/issues/5407
to show it put a background-color: red on the html element, and slide horizontally.

The smaller the viewport, the bigger the area on the right is visible. Until the scrollbar appears. and it gets back to just the width of the scrollbar. It could be a bug in Firefox related to scrollbars (with dynamic appearance).

[karlcow]

ah duplicate of #1491