bug#54450: 29.0.50; redisplay--pre-redisplay-functions issue

[Eli Zaretskii]

> Date: Fri, 18 Mar 2022 18:42:52 +0100
> From: Ergus <spacibba@aol.com>
> Cc: 54450@debbugs.gnu.org
> 
> >+  if (highlight_nonselected_windows
> >+      && !NILP (Vtransient_mark_mode)
> >+      && !NILP (BVAR (current_buffer, mark_active)))
> >+    bset_redisplay (current_buffer);
> >+
> >   /* Set TO to the interval containing the char after CHARPOS,
> >      and TOPREV to the interval containing the char before CHARPOS.
> >      Either one may be null.  They may be equal.  */
> 
> Hi Eli:
> 
> Yes, this could solve the issue for the region, but not in general for
> redisplay--update-cursor-face-highlight which was the initial problem
> with Juri's code. Same for any other function that goes into the
> pre-redisplay-functions hook.

I asked for a test case, and this is what you gave me.

If there's another test case, please show it.

> The region example reported in this issue was only the simplest way I
> found to give you a simple reproducible snippet, but the issue is a bit
> more general.
> 
> With the cursor-face we may have a similar issue cause such hook works
> in the same way, so this condition will need to be extended and extended
> and extended on every case for every function in
> pre-redisplay-functions hook.

I need to understand the more general case in practical terms.  Then I
can think about a solution.