bug#12906: 24.2.50; Parts of the screen are completely white

[Eli Zaretskii <eliz@gnu.org>]

> From: "Fabrice Niessen" <fni@missioncriticalit.com>
> Date: Fri, 16 Nov 2012 16:04:44 +0100
> 
> I sometimes have parts of the screen "undisplayed" such in the
> example at http://screencast.com/t/qLMxmSLxYvJ8.
> 
> In the beginning, all the line was white (even the fringe!).
> 
> While moving the cursor around, some characters are redisplayed correctly.
> 
> Paging up and down makes the display correct again.

This is a clear sign that some redisplay optimization is applied when
it shouldn't be.  IOW, Emacs mistakenly thinks the display does not
need to be redrawn.

> However, this is not occurring often, and not reproducible per se -- unluckily.

When that happens next, try typing "M-x".  Just "M-x", without any
command.  If that doesn't help, try "M-x redraw-display RET".

More importantly, post the result of "C-h l" (that's ell, not the
digit one), and try to remember what were you doing immediately before
the incident.  Also, in what mode was the current buffer.