test install with latest beta (but this appears on earlier versions also)
2 sidebars, fixed layout, custom width; 1040, 720-160+160
seems to cut anything beyong 1000px? or did I oversee something.
only with mobile browsers like iphone/ipad, android chrome and android native browser.
also the headerlines come out of the box.
chrome mobile renders the page at 1000px, iphone zooms to a perfect fit.
this is with default sidebar layout: Theme sizes and margins