Convert the returned position from document coordinates to viewport coordinates.

getElementPosition.js

function getElementPosition ( element ) {
        
    var x = 0,
        y = 0;

    for ( var e = element; e != null; e = e.offsetParent ) {
            
        x += e.offsetLeft;
        y += e.offsetTop;

    }

    for ( var e = element.parentNode; e != null && e.nodeType == 1; e = e.parentNode ) {
            
        x -= e.scrollLeft;
        y -= e.scrollTop;
               
    }

    return { x : x, y : y };

}