Subject: bug#61667: 29.0.60; Failure to redisplay

From: Dmitry Gutov <[email-masked]>
Date: Thu, 23 Feb 2023 18:46:38 +0200

On 23/02/2023 15:58, Gregory Heytings wrote:
> 
>>> Can you perhaps try to go further back in time (Emacs 25, 26 and 27)?
>>
>> 27 and 26 -- reproduced.
>>
> 
> Thanks!  So it looks like an old bug... but given that you did not see 
> it until recently, it could very well be an underlying change in a library.

Looking at it now, it might have been the story of the boiling frog.

The most prominent case of this problem is when the display is not 
updated for a while, for 1-2 seconds or more. But the less apparent 
scenario is when the delay between the title bar update and the buffer 
display is on the order of 100-200ms. And I've probably been seeing this 
one for a while. Just chalked it up to disk latencies, or GC, or whatever.

>> if I manage to bisect my config to find something relevant, I will 
>> report back, of course.
>>
> 
> That was the next thing I was about to suggest: try to bisect your 
> config to produce a MRE.

Here's one repro:

emacs -Q --eval "(tool-bar-mode -1)" --eval "(menu-bar-mode -1)" --eval 
"(scroll-bar-mode -1)" --eval "(global-set-key \"a\" (lambda () 
(interactive) (find-file \"test.c\")))"

Where "test.c" is the name of a file in the current dir. Different files 
can work, but for some the repro doesn't happen, like those, apparently, 
which start with a paren (which makes show-paren-mode trigger its own 
redisplay).

So, to repro:

- Run the command above
- Press "a"
- Look for the delay between the title bar and the window updates

With the above 'emacs -Q' it's not as prominent as with my config, but 
it can reach what looks like 100-200ms. Once every 10 tries or so.

This particular scenario, however, I haven't been able to repro with 
Emacs 25 or 26 or 27. It does reproduce with Emacs 28 and 29.

The more complex one (starting with 'C-x b' and using 
ido-use-virtual-buffers) still reproduces with Emacs 26, though.