www/src/py_dom.js
function $getMouseOffset(target, ev){
    ev = ev || _window.event;
    var docPos    = $getPosition(target);
    var mousePos  = $mouseCoords(ev);
    return {x:mousePos.x - docPos.x, y:mousePos.y - docPos.y};
}
function $getMouseOffset(target, ev){
    ev = ev || _window.event;
    var docPos    = $getPosition(target);
    var mousePos  = $mouseCoords(ev);
    return {x:mousePos.x - docPos.x, y:mousePos.y - docPos.y};
}
cross-browser utility functions