bug#15973: 24.3.50; Cursor switching brutally several lines at a time with next/previous-line

[Eli Zaretskii]

> From: Thierry Volpiatto <thierry.volpiatto@gmail.com>
> Date: Mon, 25 Nov 2013 15:24:18 +0100
> 
> since some days, after working some time in a buffer (undefined), 
> when using C-n/p or arrows up/down my cursor is jumping several lines at
> a time, sometimes 2 lines sometimes 5 or more, it is undefined.

Can you try to figure out when this started to appear, or (better)
bisect it?

> After killing the buffer the bug disappear, but come back after some
> time.

I suspect this is related to cache-long-scans being non-nil.  Try
setting it to nil and see if the problem disappears.  If it does, do
try bisecting or finding a reproducible recipe.

Thanks.