It's a bizarre policy to allow users to inject their personal hooks
willy-nilly.

To the extent I can incur further ill-will, I'll remind the maintainers
feature-flag proliferation is a programming tradition you should
work against, not encourage.  If you're convinced narrowing within
a critical section of redisplay is safe (and I suspect it is), do
it all the time, not just when "long_line_optimizations_p" is true.

Man up, is what I'm saying.  I wish I had your user base to vet my
larger changes.