If you navigate a menu using keyboard and mouse you can end up with more than one menu item looking selected. The cause of this is in css we have:

.menu-item:selected, .menu-item:hover, .menu-item:focused {

And mouse navigation is effecting "hover" and keyboard navigation is effecting "focused" so both can cause a item to look selected. It seems that we never set a pseudo class of "selected". What we should be doing is feeding both the mouse and keyboard interaction into the selected property and then styling css based on "selected" and remove the "hover" and "focused" css styles.