gistfile1.js

var hiders = $$('.hider').invoke('setStyle', 'z-index: 500');
var overlay = new Element('div');
overlay.setStyle({
  position: 'fixed', backgroundColor: 'rgba(0,0,0,0.7)',
  zIndex: '499', top:'0', left: '0', width: '100%',
  height: '100%', display: 'none'
});
$(document.body).insert(overlay);
setInterval(function(){
  var visible = hiders.any(function(el){
    return el.getStyle('visibility') != 'hidden';
  });
  overlay[visible ? 'show' : 'hide']();
}, 10);