bug#27350: #27350 25.2; Make `inhibit-compacting-font-caches' customizable

[Eli Zaretskii]

> From: "Felix E. Klee" <felix.klee@inka.de>
> Date: Thu, 15 Jun 2017 13:53:32 +0200
> 
> Hi there! I am the guy who asked on StackExchange. Let me know if there
> is anything that I can do to help you reproduce the bug.

Thanks for showing up.

> Perhaps the fonts that are on my system are an issue. Browsing through
> the characters in [unicode.txt][1] and with ‘xub-mode’ enabled, I find
> four fonts:
> 
>  -- -outline-Consolas-normal-normal-normal-mono-16-*-*-*-c-*-iso8859-1
> 
>  -- -outline-BatangChe-normal-normal-normal-mono-16-*-*-*-c-*-gb2312.1980-0
> 
>  -- -outline-MS Gothic-normal-normal-normal-mono-16-*-*-*-c-*-jisx0208*-*
> 
>  -- -outline-Symbola-normal-normal-normal-serif-16-*-*-*-p-*-iso10646-1
> 
> If there is a way to list all the fonts used in a file, I’m curious to
> hear about it.

It should be easy to write a function that goes over all the face
changes in a buffer and adds the faces' fonts to some list.  However,
what we really want is not the list of all the fonts, but only those
fonts which slow down redisplay.  So what I suggest is to move point
slowly through the buffer, notice the positions where it takes Emacs
significantly more time to update the cursor position, and then look
at the fonts only in those places.

(My guess would be that Ming-Liu, Batang-Che, and perhaps also Gothic
are the offending fonts.)

FWIW, on my system most of the funny characters are displayed using
Arial Unicode MS, and I see no perceptible slowdown.  Maybe you could
try setting up Emacs to do the same, and see if the problem goes
away.  Let me know if you need guidance for how to set that up.

Thanks.