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

[Gregory Heytings]

> The buffer->text->monospace flag is likewise implemented based on 
> simplistic assumptions: that only text properties could violate the 
> "monospace-ness" attribute.  Here's why this is simplistic:
>
> . faces can be applied by overlays as well
>
> . faces can be applied directly by C code
>
> . faces can be applied by using glyphs from display-tables
>
> . even if we only have the default face, fonts used for various 
> non-ASCII characters can have different metrics from the default face's 
> font, and in some cases they can even be variable-pitch fonts
>
> . portions of display can be made invisible by selective-display, not 
> just by 'invisible' properties
>
> (I'm not claiming that the above is the exhaustive list of all the 
> reasons that the assumptions in behaved_p could be violated; there could 
> be more of them.)

FWIW, the simplest recipe for the fourth point above is to create a buffer 
with a long line (with say C-u 10000 a) and to insert, somewhere in the 
middle of that line, characters which have a width equal to zero (e.g. C-x 
8 RET zero width space RET) and characters which have a width equal to two 
(e.g. C-x 8 RET e i s s a).  You'll see that your algebraic calculations 
do not work anymore.