bug#61667: 29.0.60; Failure to redisplay

[Dmitry Gutov <dgutov@yandex.ru>]

On 24/02/2023 14:56, Po Lu via Bug reports for GNU Emacs, the Swiss army 
knife of text editors wrote:
> Dmitry Gutov<dgutov@yandex.ru>  writes:
> 
>> I haven't tried this one yet (busy bisecting), but I can report that a
>> window behind Emacs, even when Emacs is not transparent, and when the
>> window is not visible, can stop the problem from happening.
>>
>> This bit me during bisecting: e.g. I can have a video in Firefox
>> playing in the background (not visible), or the Telegram window open
>> (not visible; no animations), and the problem goes away.
>>
>> This probably contributed to not having this bug reported sooner as well.
> This is very likely a bug in GNOME!
> 
> A serious one at that.  Please report it to their developers, preferably
> with the output of GNOME Shell run with the environment variable
> ``MUTTER_DEBUG'' set to 1.

I vaguely recall them talking about such problem and working on it, from 
certain dev blogs. Though I though it was supposedly fixed in GNOME 43.1 
(which I'm using).

But how does it relate to our situation? If GNOME refreshes windows more 
often that it has to, then it's a performance problem for them 
(re-rendering takes cycles), but not a correctness problem.

The only things it should do to us, is helping to mask our problem (when 
Emacs doesn't refresh quickly enough) by forcing additional repaints.

> But before you do so, please try the following:
> 
>    - Use a less resource intensive testing program (not Firefox or
>      Telegram Desktop) such as ``xclock -update 1''.
> 
>    - Update to the latest version of GNOME Shell.

I can reproduce the bug when the Emacs window covers xclock.