bug#14595: Erroneous composition of lambda in emacs-lisp buffers with prog-prettify-symbols enabled

[Eli Zaretskii <eliz@gnu.org>]

> From: Ted Zlatanov <tzz@lifelogs.com>
> Date: Sat, 15 Jun 2013 04:21:44 -0400
> 
> On Wed, 12 Jun 2013 05:18:59 +0200 Juanma Barranquero <lekktu@gmail.com> wrote: 
> 
> JB> If I visit lambda.el as in the example above, and scroll down the
> JB> buffer, I get 26 such failures, always at the same lines (168, 280,
> JB> 364, 432, 489, 538, 581, 619, 654, 686, 715, 742, 767, 791, 813, 834,
> JB> 854, 873, 891, 908, 924, 940, 955, 970, 984 and 998); however, if upon
> JB> visiting the file I go to the end of the buffer and scroll up, I get
> JB> many more failures (about 60) at diferent lines, but also consistently
> JB> the same lines.
> 
> Ouch.  I don't think I'm doing anything strange in the fontification
> keywords that implement the new prettification functionality, but why
> wasn't this seen before?  The scroll behavior probably indicates a
> display engine issue.

What issue would that be?

The problem here is that the composition rule is generated
incorrectly.