Um exemplo de uso da função hitTest do site JSFromHell.com.
Implementei com base nela um esquema de Tooltips, baseado em um script que não me recordo onde peguei.

Com isso as selects que estiverem atrás dessa tooltip somem quando ela é mostrada.

Esse esquema transforma atributos title="" de elementos <a> em tooltips.

IM = {}
/* Utils
 ***************************************************************************/
IM.Utils = {
  hitTest: function(o, l){
    function getOffset(o){
      for(var r = {l: o.offsetLeft, t: o.offsetTop, r: o.offsetWidth, b: o.offsetHeight};
        o = o.offsetParent; r.l += o.offsetLeft, r.t += o.offsetTop);
      return r.r += r.l, r.b += r.t, r;
    }
    for(var b, s, r = [], a = getOffset(o), j = isNaN(l.length), i = (j ? l = [l] : l).length; i;
      b = getOffset(l[--i]), (a.l == b.l || (a.l > b.l ? a.l <= b.r : b.l <= a.r))
      && (a.t == b.t || (a.t > b.t ? a.t <= b.b : b.t <= a.b)) && (r[r.length] = l[i]));
    return j ? !!r.length : r;
  },
  setOpacity: function(el, op){
    el.style.filter="alpha(opacity:"+op+")";
    el.style.KHTMLOpacity="0."+op;
    el.style.MozOpacity="0."+op;
    el.style.opacity="0."+op;
  },
  showElement: function(x) { // show element ( object element )
    x.style.display = "block";
    if(!x.ieFix)
        x.ieFix = IM.Utils.hitTest(x, d.getElementsByTagName("select"));
    for(var i = x.ieFix.length; i; x.ieFix[--i].style.visibility = "hidden");
  },
  hideElement: function(x) { // hide element ( object element )
    x.style.display = "none";
    if (x.ieFix)
      for(var i = x.ieFix.length; i; x.ieFix[--i].style.visibility = "visible");
  }
};


/* Tooltip
 ***************************************************************************/
IM.Tooltip = {
  init: function(id) {
    var links,i,h;
    if(!d.getElementById || !d.getElementsByTagName ) return;
    if(!$(id)) return;
    h = IM.DOM.cE("span");
    h.setAttribute("id",'im-tooltip');
    h.style.position="absolute";
    if ( $('im-tooltip') ) Element.remove("im-tooltip");
    d.getElementsByTagName("body")[0].appendChild(h);
    if(id==null) links=d.getElementsByTagName("a");
    else links=($(id)).getElementsByTagName("a");
    for(i=0;i<links.length;i++){
      this.load(links[i]);
    }
  },
  load: function(el) {
    var tooltip,t,b,s,l;
    t=el.getAttribute("title");
    if(t==null || t.length==0) return;
    el.removeAttribute("title");
    tooltip=this.cE("span","tooltip");
    s=this.cE("span","top");
    s.appendChild(d.createTextNode(t));
    tooltip.appendChild(s);
    b=this.cE("b","bottom");
    tooltip.appendChild(b);
    IM.Utils.setOpacity(tooltip, '90');
    el.tooltip=tooltip;
    el.onmouseover=this.sT;
    el.onmouseout=this.hT;
    el.onmousemove=this.pos;
  },
  sT: function(e) { // showTooltip
    var x = $('im-tooltip');
    x.appendChild(this.tooltip);
    IM.Tooltip.pos(e);
  },
  hT: function(e) { // hideTooltip
    var d = $('im-tooltip');
    if(d.childNodes.length>0) d.removeChild(d.childNodes[0]);
    if (d.ieFix)
      for(var i = d.ieFix.length; i; d.ieFix[--i].style.visibility = "visible");
  },
  cE: function(t,c) {
    var x = d.createElement(t);
    x.className = c;
    x.style.display="block";
    return(x);
  },
  pos: function(e) {
    var posx=0,posy=0;
    if(e==null) e=window.event;
    posx = Event.pointerX(e);
    posy = Event.pointerY(e);
    var x = $('im-tooltip');
    x.style.top=(posy+10)+"px";
    x.style.left=(posx-20)+"px";
    if(!x.ieFix)
        x.ieFix = IM.Utils.hitTest(x, d.getElementsByTagName("select"));
    for(var i = x.ieFix.length; i; x.ieFix[--i].style.visibility = "hidden");
  }
}

[UPDATED CODE AND HELP CAN BE FOUND HERE]


@REQUIRES Event-Listener

//Requires http://www.jsfromhell.com/Geral/event-listener

//+ Jonas Raoni Soares Silva
//@ http://jsfromhell.com/dhtml/tooltip [v1.0]

ToolTip = function( toolTipClass, attributeName, followMouse ){
	var i, o = this;
	o.s = ( o.t = document.body.appendChild( document.createElement( "div" ) ) ).style;
	( o.s.display = "none", o.s.position = "absolute", o.t.className = toolTipClass, o.a = attributeName, o.f = followMouse );
	for( i in { mouseout: 0, mouseover: 0, mousemove: 0 } )
		addEventListener( document, i, function( e ){ o[e.type].call( o, e ); } );
};
with( { p: ToolTip.prototype } ){
	p.mouseout = function(){
		this.s.display = "none";
	};
	p.mouseover = function( e ){
		for( var d = document, x = e.target; !x.getAttribute( this.a ) && ( x = x.parentNode ) != d; );
		if( x == d )
			return;
		( this.s.display = "block", this.s.top = e.clientY + "px", this.s.left = e.clientX + "px", e.stopPropagation() );
		this.t.innerHTML = x.getAttribute( this.a );
	};
	p.mousemove = function( e ){
		if( !this.f )
			return;
		this.s.top = e.clientY + "px";
		this.s.left = e.clientX + "px";
	};
}

Example

<span style="color:red;" help="aaaaaaaaaaa :)<br /><br /><b>uuuuuuuuuu :(</b>">OIIIIIIIIIIII, PASSA A MÃOZINHA EM MIM XD</span>
<span style="color:green;" help="nooossa !!!!<br /><br /><b>affffffff iiii</b>">LALAAAAAAAAAAAAAAAA</span>

<script type="text/javascript">
//<![CDATA[

new ToolTip( "tooltip", "help", true );

//]]>
</script>