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

[Ergus via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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.