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

[Gregory Heytings]

> xdisp.c[9319,9369]: Calculates dy algebraically without char-by-char.
>
> xdisp.c[9537,9577]: Calculates CAPPED to limit char-by-char scan for 
> moving backwards (Blandy and Moellmann wrote xdisp like a singly linked 
> list -- fast and precise forwards, much less so going backwards).

Thanks, finally we get an indication of what your code does and where.

I looked at it briefly, and the first thing I saw does not seem promising. 
Your algebraic calculations are enabled when behaved_p, which depends on 
buffer->text->monospace.  That field is set (unconditionally) to true in 
Fget_buffer_create, and reset only in only one place, in add_properties. 
Can you please elaborate a bit and explain why doing that would be TRT?