bug#11484: 23.4; Scrolling leaves traces of old text behind

[Eli Zaretskii]

> Date: Sun, 21 Oct 2012 22:11:36 -0700
> From: Daniel Colascione <dancol@dancol.org>
> CC: Eli Zaretskii <eliz@gnu.org>, 11484@debbugs.gnu.org
> 
> Are displays still slow enough that we have to care about graphics 
> optimization
> at this level?

As Stefan points out, yes, even on X11.

There's also another annoying result of frequent full redrawing:
flickering.  I suspect that disabling this optimization will have the
effect that leaning on the left or right arrow key would cause the
line with the cursor flicker due to constant redrawing.  Similar
problems will show up with the mode line, which is updated quite
frequently.