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

[Eli Zaretskii]

> From: dick <dick.r.chiang@gmail.com>
> Cc: 57669@debbugs.gnu.org
> Date: Sat, 10 Sep 2022 08:51:50 -0400
> 
> 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.

Btw, the way you reset the monospace flag in add_text_properties is
also problematic: the code is looking only at the specific face that
is the value of the property.  But Emacs applies faces by merging them
with other sources of face information, not just by using the single
face named by the property.  As the simplest example, consider a face
whose only non-unspecified attribute is :inherit -- in that case,
AFAIU your code will not reset the monospace flag, but when Emacs will
actually use the face for rendering, it will follow the inheritance
chain and might find there one of the face attributes your code does
care about.

Also, I don't see the monospace flag being reset if the face's font is
not a fixed-pitch one.  Did I miss something?