bug#51490: Show an indicator when Emacs is busy somewhere in the Emacs window

[Eli Zaretskii]

> From: Stefan Kangas <stefankangas@gmail.com>
> Date: Wed, 21 Sep 2022 05:34:51 -0700
> Cc: 51490@debbugs.gnu.org
> 
> Lars Ingebrigtsen <larsi@gnus.org> writes:
> 
> > Eli Zaretskii <eliz@gnu.org> writes:
> >
> >> Mode line is drawn in C, not in Lisp.
> >>
> >> And if the problem is that we cannot run the display code, either,
> >> then how would it help to poke the glyph?  It won't be shown on the
> >> glass, because redisplay cannot run.  Right?  Or what am I missing?
> >
> > Redisplay can run, but we can't run any Lisp code, and the normal
> > formatting of a mode line does (potentially) run lots of Lisp code,
> > doesn't it?
> 
> BTW, I think one goal here should be to optionally replace the glyph
> with a sequence of images/icons (preferably SVG files).

These are also (special kinds of) glyphs.