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

[Gregory Heytings]

> > On GUI frames, can't we do something similar to what we do for fringe 
> > bitmaps?
>
> I'm not sure I follow.  Fringe bitmaps are drawn by redisplay, and they 
> do access faces, text properties, overlays,...  So how would this solve 
> the problem?

Aren't they the simplest graphical element drawn by Emacs, a simple 
black-and-white bitmap?  I don't think a "busy" indicator would have to 
access faces, text properties and overlays, it could be a simple (list of) 
black-and-white bitmaps drawn at a fixed position, e.g. on the left of the 
mode line.  The interaction with Lisp would be minimal.