bug#56393: Actually fix the long lines display bug

[Drew Adams]

(Caveat: I'm not following this thread.  Ignore
if not helpful.)

> narrow-mode (ab)uses a feature in a way that...

If this is really about long-lines display, could
you please consider not using a name such as
`auto-narrow-mode' or `narrow-mode' for this?

Functions and variables with `narrow' in the name
should generally, when possible, be reserved for
the usual feature of narrowing a buffer.  (And in
particular for users doing that.)

If the reason for narrowing is only to carry out
or enable some other action/behavior/feature (e.g.
something to do with long-lines display?) then
maybe put that in the name, without `narrow'?

Will users look for this mode when they want to
narrow a buffer for some reason?  Or is this just
something that has to be done in order to effect
something else - a precondition/dependency?

IOW, is this the best name, for users?  What will
a user looking for functions for narrowing think
when coming across `narrow-mode'?  If this isn't
something users will generally use for narrowing
a buffer then at least please make that clear in
the doc string.