Add an overflow option/setting to the menu control. If the height or width of a menu group exceeds the current page viewport height or width, a new section would be added to the beginning and end of the menu to allow scrolling without adding scrollbars. 

Example - If a horizontal menu has a sub menu that exceeds the height of the current page viewport, add sections to the top and bottom of the sub menu to allow the menu items to be scrolled up or down (mouseover or click) without adding a scrollbar to the sub menu.