code: function handleMouseOverOut (handler, event) {
    const target = event.currentTarget
    const related = event.relatedTarget || event.nativeEvent.toElement
  
    if (!related || (related !== target && !contains(target, related))) {
      handler(event)
    }
  }
docs: ---
  category: utilities/DOM
  ---
  
  Simple implementation of mouseEnter and mouseLeave.
  React's built version is broken: https://github.com/facebook/react/issues/4251
  for cases when the trigger is disabled and mouseOut/Over can cause flicker moving
  from one child element to another.
  
  @param handler {function} Callback function for handling the event
  @param event {Event} The DOM Event that was fired