Hides/shows a fixed bar element on a page. Accepts a DOM node as ELEMENT. Y can be adjusted as needed.

fixedbar.js

(function(ele){
  var win = window,
		addEvt = win.addEventListener,
		y = ele.offsetHeight;
	
	ele.style.transition = "0.2s";
	
	function hide () {
		ele.style.opacity = 0.1;
	}
	
	function show () {
		ele.style.opacity = 1;
	}
	
	addEvt( "scroll", function() { 
		if ( win.scrollY > y ) {
			hide();
		}
		else{
			show();
		}
	});
	
	ele.addEventListener("mouseover", show);
	ele.addEventListener("mouseout", hide);
	
	if ( win.scrollY > y ) {
		hide();
	}
})(ELEMENT)