bug#45184: 26.1; Private Unicode plains 15 and 16 seem not supported

[Eli Zaretskii]

> From: jsbien@mimuw.edu.pl (Janusz S. Bień)
> Cc: 45184@debbugs.gnu.org
> Date: Sat, 12 Dec 2020 09:43:53 +0100
> 
> >> > That's because Emacs has no idea that you are using SPUA planes for
> >> > Polish.
> >> 
> >> The fact that I want to use the characters for Polish is irrelevant.
> >
> > Not the way Emacs looks up fonts installed on the system.  It uses the
> > character's script and then a database of representative script
> > characters to find appropriate candidate fonts.
> 
> Where I can find more details about the process?

Unfortunately, mainly in the sources: font.c, fontset.c, and
fontset.el are the main places to look.  This area is notoriously
under-documented in Emacs.