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

[Ergus]

On Fri, Mar 18, 2022 at 08:38:18PM +0200, Eli Zaretskii wrote:
> > 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.
Hi Eli

A similar issue happens if in the test buffer you do:

M-x cursor-face-highlight-mode
(setq cursor-face-highlight-nonselected-window t)
M-: (insert "AAA"(propertize "Hello World" 'cursor-face 'highlight)"BBB")

Then try to move the cursor over the inserted text with the arrows (you
should see the highlight when cursor is over the propertized text); The
try te same and from other buffer (using with-selected-window).

In the second case you will see the cursor moving but the highlighted
not updated.

> > 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.