I had some worning error. It's not so dangerouse.

In next code we can take a null parent so null doesn't have a method .parentNode.

var withinElement = function( event ) {

Check if mouse(over|out) are still within the same parent element

var parent = event.relatedTarget;

Firefox sometimes assigns relatedTarget a XUL element

which we cannot access the parentNode property of

try {

Chrome does something similar, the parentNode property

// can be accessed but is null.

if ( parent !== document && !parent.parentNode ) {

return;

}