bug#57669: 29.0.50; C-n, C-p off under long lines

[dick]

You've identified the weakest point and you continue to squeeze.
Fortunately my weakest point is monospace_p, an heuristic, one which
I feel makes the right compromises.

I wouldn't be surprised if "heuristic" didn't surface in your self-study
of programming.  The narrowing in 29.0.50 is also an "heuristic."

> As the simplest example, consider a face whose only non-unspecified

Like you, I largely dismiss prose but will begrudgingly run a minimum
reproducible example.  If it's so simple, please follow your own
bug-reporting advice:

  Please describe exactly what actions triggered the bug...
  If you can, give a recipe starting from 'emacs -Q'

You'll find that I *always* do this in epic arguments of this scale.
You, I noticed, traffic in hand-wavy refrains like "I've been doing this
for years," "If you read the code, you'd know why," "He simply doesn't
understand the code."