bug#66398: 29.1; Buggy handling of transparency changes / blur/unblur

[Eli Zaretskii]

> From: Tollef Fog Heen <tfheen@err.no>
> Date: Sat, 07 Oct 2023 22:07:15 +0200
> 
> 
> Hi folks, this is Debian bug #1053298,
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1053298 :
> 
> In my setup, I make non-focused windows semi-transparent (using Xmonad
> and its compose support).  This used to work perfectly, but with the
> recent-ish update of emacs from 28.2 to 29.1, this broke.  (Basically,
> what's described on
> https://hackage.haskell.org/package/xmonad-contrib-0.16/docs/XMonad-Hooks-FadeInactive.html)
> 
> I've bisected this down to:
> 
> commit b299f173490f5c51476ad3c8436b19bb091c1b00
> Author: Po Lu <luangruo@yahoo.com>
> Date:   Tue May 10 09:32:59 2022 +0800
> 
>     Update alpha frame parameter when the window manager changes it
> 
>     * src/xfns.c (x_set_alpha): New function.  Set
>     `alpha_identical_p' flag.
>     (x_frame_parm_handlers): Use it to handle `alpha' instead.
> 
>     * src/xterm.c (x_set_frame_alpha): Make tests against current
>     alpha safer.
>     (handle_one_xevent): Set frame alpha when alpha property
>     changes.
>     * src/xterm.h (struct x_output): New flag `alpha_identical_p'.
> 
>  src/xfns.c  | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  src/xterm.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++--
>  src/xterm.h |  4 ++++
>  3 files changed, 108 insertions(+), 3 deletions(-)
> 
> which seems like a plausible change for causing such a bug.
> 
> Note that the transparency of the window corrects itself if I switch
> buffers, and in some cases if there's just a message in the minibuffer,
> so it seems like emacs isn't always picking up the change in
> transparency which happens as it is being focused, but on a buffer focus
> change, it catches up.
> 
> Interestingly enough, this only reproduces for me with the nvidia X11
> driver, on my laptop (with Intel graphics) with the same emacs setup,
> I've never seen it.
> 
> In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.38,
>  cairo version 1.16.0) of 2023-08-30, modified by Debian built on
>  x86-csail-01
> Windowing system distributor 'The X.Org Foundation', version 11.0.12101008
> System Description: Debian GNU/Linux trixie/sid

Po Lu, any comments?  If possible, I'd like to fix this in Emacs 29.2.