Don’t you think that’s a bit odd?….to allow most of the site to appear ‘boxed’ but to have one part of it as an immovable full-width section? I don’t see the point of a site width slider if your site, all of it, can’t be the chosen width.
I guess the alternative is to hand code the width for that top bar (where? what’s the selector?), or just remove it altogether.
Kind of spoils the ship for a ha’pence of tar, as they say.