MooTools - Sticky menu

#bar{width:100%;background:#333;color:#fff;padding:20px}#con{width:200px;height:1000px;background:#666}
window.addEvent(domready, function() {if($(bar) != null){new NaMooSticky();}}); 

var NaMooSticky = new Class({initialize: function(){this.bar = $(bar);this.position_saver = this.bar.getPosition().y;window.addEvent(scroll,this.action.bind(this));},action: function(){if (this.position_saver< window.getScroll().y){this.bar.setStyles({position:fixed,top:0});}else{this.bar.setStyles({position:relative,top:0});}}});
<div id=bar>menu</div><div id=con></div>