bug#61667: 29.0.60; Failure to redisplay

[Dmitry Gutov]

On 24/02/2023 15:46, Eli Zaretskii wrote:
> > Date: Fri, 24 Feb 2023 15:12:53 +0200
> > From: Dmitry Gutov<dgutov@yandex.ru>
> > Cc: Po Lu<luangruo@yahoo.com>,61667@debbugs.gnu.org,
> >   Eli Zaretskii<eliz@gnu.org>
> >
> > 817dd546497aadefbe9acc8762e3f7190799c5e6 is the first bad commit
> > commit 817dd546497aadefbe9acc8762e3f7190799c5e6
> > Author: Stefan Kangas<stefan@marxist.se>
> > Date:   Sun Sep 13 18:24:31 2020 +0200
> >
> >       Improve frame-title-format and icon-title-format
> >
> >       * src/xdisp.c (syms_of_xdisp): Replace 'invocation-name' with the text
> >       "%b - GNU Emacs" and replace "@" with " at ".  (Bug#41147)
> >       * etc/NEWS: Announce the above change.
> >
> >    etc/NEWS    | 11 +++++++++++
> >    src/xdisp.c |  3 +--
> >    2 files changed, 12 insertions(+), 2 deletions(-)
> >
> > Triple-checked that as well: the commit before it doesn't reproduce the
> > above scenario, and this one does.
> >
> > Looking at the commit, there is another difference in behavior too:
> >
> > - With this commit, all is as described previously: I press 'a', window
> > title changes, there is a delay (randomly), then the window contents change.
> >
> > - Before this commit: the window title doesn't change, it's always
> > emacs@hostname. But when I press 'a' (bound to 'find-file' lambda),
> > there never is a noticeable delay before the window contents change. The
> > buffer is displayed instantly.
> How is this consistent with your previous finding that the problem
> exists in Emacs 25, 26, and 27.  The change above is only present in
> Emacs 28.  Does this mean that the problem 100-200ms delay and the
> original problem are two different problems?

Easy: my configuration contains a customization for frame-title-format.

It's set to

  (setq frame-title-format '(buffer-file-name "%f" ("%b")))

All the time I spend bisecting the config I didn't think to change it 
(it's the very first line). And this makes the problem appear with Emacs 
27 and 26 too.

The only question that's still not clear is why this causes a 
multi-second delay with my personal config, and only 200-300ms with 
'emacs -Q'. But changing the value fixes both.

> Anyway, if the changes in the frame's title are somehow related to
> this, their effect is to cause Emacs to call x_set_name_internal to
> display the new title.  Could it be that this function takes such a
> long time to execute?  Or does it have some strange effect on the WM?

My vague (and likely wrong) guess would be that the WM knows it needs 
some update from the Emacs window, gets it from the title bar before 
everything else, and marks the update as completed.