When I deactivate "Show Page Headers" in "Customize > Content Options" the total page width becomes wider, i. e. more than 100% of my browser window, when my window is maximized down to a certain width (at which it becomes normal again), showing the horizontal scrollbar.
I tested it using:
Mac OS 10.8.5
Chrome and Safari
MBA at screen resolution 1440x900
Here is an example of a site with deactivated "Show Page Headers": http://5.site11.com/.
Thank you in advance for any help! :)