Comparing 13.51 to 13.50 with the official regtest checker shows a plethora
of minor changes - clearly something in the spacing engine has changed
enough to flags lots of errors. I was looking at theses regtests as light
relief from my pixel comparator, which shows 277 changed images (from 13.49,
FWIW). TBH, there are so many changes on the official regtest page
(http://lilypond.org/test/v2.13.51-1/compare-v2.13.50-1/index.html) that
it's not feasible to do a good comparison. I'll look over my pixel
comparator, since that flags minor changes more clearly, but would welcome
thoughts as to what to do about the regtests for this version.