I think your enhancement is great. However, there are problems, if one of the resizeable's parent nodes is positioned ''relative'' or ''absolute''. In that case the left and top offsets of these parent nodes must be subtracted from the left- and top-value that will be used for the containment calculations (i.e. the first to elements of ''options.containment'' ).
If the parentNode itsself is positioned ''relative'' or ''absolute'' everything's quite easy, then options.containment should just look like (around line 65):
options.containment = [
(ce.offsetWidth || ce.scrollWidth),
(ce.offsetHeight || ce.scrollHeight)
However if one of the other parent nodes are positioned things get more difficult I guess.
At the moment I have no really good idea how to fix this properly...