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

[dick]

EZ> So I think this part of the code is equivalent to what we do, in
EZ> back_to_previous_visible_line_start

I'm glad you noticed!  It means you're actually reading the code!

One difference is I gate line scanning (as opposed to character
scanning) with behaved_p, whereas GNU crosses its fingers and hopes its
POS_LIMIT estimate is far enough.  So oddly enough, for all my cavailier
assumptions, at least this part of the code [xdisp.c 9537:9577] is
*more* conservative than GNU.