Re: Suggest installing more fonts?

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Gregory Heytings <ghe@sdf.org>,  larsi@gnus.org,  emacs-devel@gnu.org
> Date: Fri, 16 Oct 2020 15:40:11 -0400
> 
> > Instead, it is much easier to have a feature implemented in Lisp that
> > would detect characters with no font glyphs and put help-echo on them.
> > We have infrastructure for such features in jit-lock.el.
> 
> Indeed.  And we can make this very efficient if the redisplay stashes
> in some Lisp var the chars-with-locations of the tofu it generates.

I think relying on such a trace variable would be unreliable, because
you can never predict which parts of the window will be actually
redisplayed, and the display engine can only leave the trace about
those parts where it tried to find a font glyph for a character.

Emacs already knows not to search the available fonts for a character
that it already tried to display and failed to find a suitable font.
We should probably reuse the information which says a character
doesn't have a font for finding such characters and marking them.  It
cannot be too expensive, I think.