AjaxControlToolkit.ModalPopupBehavior.prototype._attachPopup = function() {
    /// <summary>
    /// Attach the event handlers for the popup to the PARENT window
    /// </summary>
    if (this._DropShadow && !this._dropShadowBehavior) {
        this._dropShadowBehavior = $create(AjaxControlToolkit.DropShadowBehavior, {}, null, null, this._popupElement);
    }
    if (this._dragHandleElement && !this._dragBehavior) {
        this._dragBehavior = $create(AjaxControlToolkit.FloatingBehavior, {"handle" : this._dragHandleElement}, null, null, this._foregroundElement);
    }
    $addHandler(parent.window, 'resize', this._resizeHandler); // <== This WORKS
    $addHandler(parent.window, 'scroll', this._scrollHandler); // <== This DOES NOT work
    this._windowHandlersAttached = true;
}

$(top.window.document.documentElement).scrollTop();

$(top.window.document.body).scrollTop();