bug#51649: 29.0.50; Emacs won't show hourglass

[Lars Ingebrigtsen]

Test case:

(progn
  (switch-to-buffer "*Messages*")
  (while t))

Ideally, this should show the "hourglass" pointer after a second to show
that Emacs is busy, but ideally not do anything else.

But instead nothing happens -- no hourglass.  Until I move the mouse
pointer, hit the keyboard, or otherwise do something to make Emacs
trigger an update.  Then I get the hourglass -- but Emacs also shows the
*Message* buffer, which it shouldn't.

The latter is due to:

         redisplay_preserve_echo_area (21);

which I added to x_show_hourglass -- but it fails to do what it was
supposed to (i.e., show the hourglass, and it has bad side effects).  So
I've now removed it.

So...  what is the way to make Emacs show the hourglass, but without
triggering any other redisplay?


In GNU Emacs 29.0.50 (build 58, x86_64-pc-linux-gnu, GTK+ Version 3.24.30, 
cairo version 1.16.0)
 of 2021-11-06 built on elva
Repository revision: 1733448994a6dea4dbc0c19c02d9d425469119ce
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux bookworm/sid


-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no