bug#9771: 24.0.90; Redisplay problems with control characters

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 17 Oct 2011 03:48:27 -0400
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 9771@debbugs.gnu.org
> 
> > 3. Long lines with display vectors make Emacs really slow (with bidi)
> 
> It's not the display vectors in general that cause this.  It's
> specifically the display of control characters.

The reason for the slowness is the bidirectional property of the
control characters, and a peculiarity of the UBA, the Unicode
Bidirectional Algorithms, regarding the processing of sequences of
characters with that particular property.  In the particular case in
point, it causes Emacs to search to the end of the long line, when it
is about to display the first null character.

So now this question becomes very relevant:

> is there some real-life use case behind your recipe?

Depending on the answer, it may or may not be a good idea to look for
some heuristics for this case before 24.1 is released, because AFAICS
the current implementation of the UBA requirements is correct.