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

[Gregory Heytings]

> > I'm thinking aloud here, sorry if it isn't useful.  But I think we have 
> > at least one similar occurrence: XTflash.
>
> Which part of it seemed similar to what is being discussed here?

The fact that a portion of the frame is modified without a complete 
redisplay of the frame, through primitive drawing functions?  Am I missing 
something?  Would it not be possible to draw a busy indicator at a fixed 
position of the frame with XDrawRectangle for the background, followed by 
XDrawPoints for the foreground?

> > But would it be really necessary to access these faces each time the 
> > indicator would be drawn?  We could for example cache the values of 
> > these two colors somewhere
>
> Faces are already cached.  But for each cache there comes a time when it 
> must be flushed and/or refreshed, and during those times you cannot 
> safely use the cache.

What I had in mind is a more primitive cache mechanism: two C variables.

> > or decide to do something different and use a specific function to set 
> > these two colors without going through the whole face machinery, 
> > something like (set-busy-indicator-colors FOREGROUND BACKGROUND).
>
> As soon as we allow set-busy-indicator-colors or somesuch, we have a 
> problem with Lisp programs doing that exactly when there's time to 
> redraw the indicator.

We could specify that set-busy-indicator-colors is only meant to be used 
in init files, for example.