bug#26333: 26.0.50; Hang when displaying some special characters for the first time

[Eli Zaretskii <eliz@gnu.org>]

> From: Steven Allen <steven@stebalien.com>
> Cc: 26333@debbugs.gnu.org
> Date: Sat, 01 Apr 2017 15:00:58 -0700
> 
> 
> So, I could have sworn I tested this with emacs -Q when I first found
> the bug (I've been sitting on it for a while) but I guess I didn't. I
> apologize.
> 
> It only reproduces for certain fonts.
> 
> Known Bad: Source Code Pro, Office Code Pro, Fira Mono
> Known Good: Symbola, DejaVu Sans Mono (the default on my system)
> 
> So, to reproduce, evaluate the following before pasting the special
> character.
> 
>   (set-face-attribute 'default nil :font "Source Code Pro-10")

Is this the only thing one has to do, in "emacs -Q", in order to
reproduce the problem with these fonts?  Or do you have other
font-related customizations on your init files?

> Eli Zaretskii <eliz@gnu.org> writes:
> > That's normal:
> 
> Hardly. 15 seconds is a *long* time for a computer and every other
> application displays the character in question instantly. Not even
> regenerating my entire font cache (fc-cache) takes that long.

I meant "normal" as in "Emacs is known to do that sometimes when
searching for a font".  I'm not sure comparison with other
applications is valid, as Emacs attempts to solve a more complex
problem, and its font-searching algorithms are biased in favor of
finding fonts which support scripts and languages, not symbols.

As for comparison with fc-cache, Emacs many times examines each font
several times with different options, so it could be slower.

> > If you install the Symbola font, I think the search will
> > be much faster.
> 
> I do have Symbola installed and the character does display correctly
> eventually.

Is it eventually displayed using Symbola or some other font?