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

[dick]

> showing your code that deals with very long lines and explaining how
> it solves that

I can't believe I'm dignifying this.  All line references from commit
beb489e.

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).

A high-level description (which I understand is worthless to you, since
you're all about code not talk) can be found on YouTube.  Cover the
right half of the screen if misshapen faces aggravate a heart condition.