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

[dick]

monospace-p I agree is an heuristic like narrowing.  In the majority of
cases of long lines, it applies the algebraic shortcut when it thinks it
can get away with it.

What if it's wrong?  Then it won't scroll up or down the full page.
What if it scrolls zero on a C-n?  That'd be a degenerate case, one
which I'd have a hard time contriving, but that would only make it as
culpable as 29.0.50 which we already know can scroll zero on C-n.

But until Commercial can gather a following, my sanguinity is all
pissing-contest speculation.