Prevent line breaks in menu

Using CSS style white-space: nowrap; in menu, the long lists
no longer get dropped to the next line (wrapping behavior).
Instead, they are allowed to overlap out of the menu.
Change overflow-y from 'scroll' to 'auto'

The goal is to prevent the display of a scrollbar in the menu
(in the middle of the page) unless actually necessary:

   "The behavior of the ‘auto’ value is UA-dependent,
    but should cause a scrolling mechanism to be provided
    for overflowing boxes."
    -- description of value 'auto'
       in 11.1. The ‘overflow’, ‘overflow-x’ and ‘overflow-y’ properties
       in CSS basic box model (css3-box)
       http://www.w3.org/TR/css3-box/#overflow-x