Malfunctioning of Sticky Header on 13-inch screens with open bookmarks bar

Hi I brought up the site online few days ago and i notice that on some 13 inches or less screens there are problems in header displaying (template 13): while scrolling the page the header assumes a width that covers all the content. This does not happen on a 15-inch screen even though the bookmark bar is open. I Attach 13-inches Macbook Air screenshots, browser safari. I think it does not depend on the browser because it also happens on pc windows with browser explorer