Use calculated index instead of DOM index

[ced-b]

There seems to be a scenario when there are multiple thead entries in a table where some versions of Firefox (around 47) return -1 for cell.parentNode.rowIndex, which winds up with the whole top part being laid out wrongly.

However, I looked this over and did not see any reason why not to use the calculated i for the row index instead.

[Mottie]

Hi @ced-b!

That is odd that it would make a row index of -1. According to spec:

As a child of a table element, after any caption, and colgroup elements and before any tbody, tfoot, and tr elements, but only if there are no other thead elements that are children of the table element.

Meaning a <table> is only allowed one child <thead> (ref)

That being said, I don't see any issue with this change.

Thanks!

[ced-b]

@Mottie Thanks. I guess I abused the thead a bit then. But I hope this update still makes your plug-in more stable.