React contour problems

[etpinard]

Split from #2624, with reproducible example and explanations in https://codepen.io/etpinard/pen/aGxMVX

cc @alexcjohnson

[alexcjohnson]

Wow, that's no good - the more you click back and forth in that pen the worse it gets, until the whole page crashes. Thanks for splitting this out and making that codepen @etpinard !