This code is the start of something that would make your
narrowed-on message remain in the same place post-narrow,
which is possibly a good idea.
(imported from commit 0a557d6492de470928245b33f95ae8c0102db0a6)
I would like to have split this into several commits, but it's hard to do
incrementally, because functions need to be defined before their use in global
data.
(imported from commit 31533396b236f3b657950b073982c317256f082a)