guard against Chromium's counter-intuitive mouseenter/mouseover/mousemove...

guard against Chromium's counter-intuitive mouseenter/mouseover/mousemove events that get generated when a scroll event moves the element under the mouse
Also, use "mousemove" instead of "mouseover" event as it properly highlights
the element under the mouse even for single pixel mouse movements. "mouseover"
would fail to highlight if the keyboard navigation had moved the highlight
to a different element within view.
These changes fix the problem of generating a chain of mouse events that
would mess up keyboard navigation when the mouse pointer happened to be over
the <ol>. These changes make the interaction between mouse and keyboard much
more like what's expected in most GUI toolkits. (The only difference now is
that the menu can't overflow the browser window, but there's nothing we can
do about that.)