@ctrlcctrlv, would you review this one? It's going to take either of us a while to process the relevant code, but you have seen more of it than I have and probably have better test cases.

Yes, I can.

God, this part of the code is such a slog and has such a complicated history, but I think I understand it. The most important commit is bbde2a5 as that's where George tried to fix this originally in 2012 and is likely a good version, but it predates certain additions like fvt_addlayer. The version @jtanx proposes seems to match in the ways that matters.

To be certain, though, I did the following runtime tests:

• lbearing to 200
• both bearings to 113
• rbearing to 90
• width to 500
• and then both back to 200

Given all that, I think this is fine to merge. It's certainly less broken than master, and I found no issues.