Absent an image loaded near the page head, the menu covers content like the breadcrumbs, top blocks, and/or page content itself, when viewed on smaller screens.
For example, a page like:
http://www.masterbootstrap.com/documentation
When on a Chromebook (1024 x 576), at 90%-150%, variable amounts of content are hidden under the menu when scrolled at top of page. That hidden content is never visible.

UPDATE: I've tested this on other computers, and in other browsers. It happens on all of them. Issue can be experienced by zooming in in any browser on devices where screen resolution large.
The padding-top for a middle sized screen is missing in css.