elementFromPoint() problems when window has been scrolled .

Hello, All ...
I have an HTML file which contains pages of formated text, all within a <p> tag, which has an onclick call to a jscript function. This function gets mouse cursor position at the click event, then passes this position to elementFromPoint(pos.x,pos.y) to return my element object, which I then highlight.

I did this using this method because it seemed to work best with my nested elements (always <span>) - sometimes nested 15-20 layers deep. This enables me to easily view the level of nesting, and start and end of this block of clicked text, which can sometimes be hundreds of lines long, within a file which can be thousands of lines long.

I got this working in IE and Firefox, which are the only 2 browsers it will ever be used in. Well, at least I thought I had it working -until I scrolled the page, and suddenly it wasn't working anymore. This bug is the same in both browsers.

function gCO(e) {
  e = e || window.event;
  var pos = getPosition(e);
  var obj = document.elementFromPoint(pos.x, pos.y);
  doHighlight(obj);
}

function doHighlight(ele) {
  if (ele.style.backgroundColor == "pink") {
    ele.style.backgroundColor='';
  }
  else {
    ele.style.backgroundColor="pink";
  }
}

function getPosition(e) {
  var cursor = {x:0, y:0};
  if (e.pageX || e.pageY) {
    cursor.x = e.pageX;
    cursor.y = e.pageY;
  }
  else {
    var de = document.documentElement;
    var b = document.body;
    cursor.x = e.clientX +
      (de.scrollLeft || b.scrollLeft) - (de.clientLeft || 0);
    cursor.y = e.clientY +
      (de.scrollTop || b.scrollTop) - (de.clientTop || 0);
  }
  return cursor;
}


<p onclick="gCO(event)">

Is there a way to compensate for my page scroll offset?

Thanks in advance for any help given,
Sean