Comment 23 for bug 1593963

personally, I think the fix needs to be made upstream, in the calculation of f.cuts[idx]. The f.cuts data is clearly bad, and should not be allowed to reach the compose() routine in the first place.
I assume that the reason it is bad is because the Bezier handles are retracted, which means that the normal first derivatives are zero, and one needs to use something like the chain rule to evaluate the response. In any event, this is an analytical problem which needs to be addressed upstream.

just my 2cents...