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

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Gregory Heytings <gregory@heytings.org>,  stefan@marxist.se,
>   51490@debbugs.gnu.org
> Date: Fri, 23 Sep 2022 17:06:05 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > It should be possible, yes.  I just said that we don't have such an
> > infrastructure yet, it needs to be written first.  When someone does
> > write it, they will have to deal with the issue of what kind of
> > drawing do we allow there; for example, a 1-pixel line is easy, but is
> > unlikely to be visually appealing.
> 
> The spinner will presumably not be done with line drawing, but consist
> of a set of 8 glyphs that are defined along the lines of fringe markers.

Then this gets back to how to do that behind the back of the display
code.  We'd need to invent new machinery for that.

> > And the main problem this will have to solve is this: whatever you
> > draw behind the back of the display engine will run the risk of being
> > wiped out when the next redisplay cycle kicks in.
> 
> Yes, we'd have to have a way to tell the redisplay to lay off redrawing
> in that rectangle, probably.

That's also something that doesn't exist, and will have to be added
somehow.