findParentBy - determines the way how a cell finds a suitable parent when it's dragged over the paper. The cell with the highest z-index (visually on the top) will be chosen. findParentBy option comes to play when the paper is in embedding mode. All possible values are 'bbox' (default), 'center', 'origin', 'corner', 'topRight', 'bottomLeft' and it also can be a function. The function accepts an element and it returns array of possible candidates.
findParentBy: function(element) {
    var bBox = element.getBBox();
    return this.getElements().filter(function(el) {
        return bBox.intersect(el.getBBox());
    });
}