Hm, yes it makes sense

Margins weren't taken into account because they doesn't work for table rows anyway, but this may be useful for div, ul/ol lists

Shall I submit a PR?

Sure, if you have time, desire and already working solution.
I'll definitely accept your PR and made additional changes by myself, if needed.

or you could drop a snippet here with required changes and I'll do the rest

I already have an idea where fix should be applied. Additional condition to check margins here using getStyle func

Here's the snippet:

    getRowsHeight: function(rows) {
      var opts = this.options,
        prev_item_height = opts.item_height;
      opts.cluster_height = 0
      if( ! rows.length) return;
      var nodes = this.content_elem.children;
      var node = nodes[Math.floor(nodes.length / 2)];
      opts.item_height = node.offsetHeight;
      // consider table's border-spacing
      if(opts.tag == 'tr' && getStyle('borderCollapse', this.content_elem) != 'collapse')
        opts.item_height += parseInt(getStyle('borderSpacing', this.content_elem), 10) || 0;
      // consider margins (and margins collapsing)
      var marginTop = parseInt(getStyle('marginTop', node), 10);
      var marginBottom = parseInt(getStyle('marginBottom', node), 10);
      opts.item_height += Math.max(marginTop, marginBottom);
      opts.block_height = opts.item_height * opts.rows_in_block;
      opts.rows_in_cluster = opts.blocks_in_cluster * opts.rows_in_block;
      opts.cluster_height = opts.blocks_in_cluster * opts.block_height;
      return prev_item_height != opts.item_height;
    },

Thanks!

But now we should additionally take into account margin collapsing, cases when those collapsing are not applied, etc..
I'll investigate it at free time, approximately at these weekend

Happy to help 😄

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:

.clusterize-extra-row{
  margin: 0;
}

But I'm not sure if its the best solution.

Thanks @zaygraveyard for your contribution!

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