As for the margin collapsing, I had already investigated this problem before for react-sortable-hoc.
And found that its a very hard problem to solve completely but if we consider that all items have the same margins then the result of margin collapsing will be a simple max operation (as in the snippet).
For more info you can check-out the comments on the PR react-sortable-hoc#27.
Oh and I forgot to mention that I added the following rule to the clusterize.css:
Your PR did all the necessary work, only thing that I needed to add was additional condition to check if it's not table row since margins don't affect them so that calculations should be made accordingly