bug#15841: Display bugs with cache-long-lines non-nil

[Eli Zaretskii]

> Date: Sat, 09 Nov 2013 13:18:29 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: michael_heerdegen@web.de, 15841@debbugs.gnu.org
> 
>  emacs -Q --eval "(setq-default cache-long-scans nil)"
>  C-x C-f src/xdisp.c
>  M-: (setq cache-long-scans t) RET
>  M-x linum-mode RET
>  M->
> 
> Many line numbers near the end of the buffer are not shown in the
> margin.

This seems to be related to font-lock: turning off
global-font-lock-mode makes the problems go away.  Manually invoking
font-lock-fontify-buffer before turning on linum-mode also eliminates
the problems, so it looks like JIT Font Lock is the prime suspect.