[Gimp-gui] GTK3 "empty black window"/"black flash" bug that affects GIMP-2.99

[Elle Stone <ellestone ninedegreesbelow com>]

Hi All,

This is just an FYI for anyone who encounters the issue and doesn't 
already know about this bug.

There is a long-standing GTK3 bug - that's recently been fixed in GTK 
git master - that does affect GIMP-2.99. I noticed it yesterday when I 
tried for the first time to use GIMP-2.99 for actual editing.

This bug causes an empty black window to briefly appear when opening 
editing dialog windows such as "Filters/Blur/Gaussian Blur".

FWIW, this GTK3 bug was reported back in 2015 for GTK3 version 3.22.x, 
and there is a trail of bug reports on various OS's running various 
desktops, including:

    Black background appears briefly before window gets drawn:
    https://bugzilla.gnome.org/show_bug.cgi?id=748498
    https://gitlab.gnome.org/GNOME/gtk/issues/550

    GTK3 windows appear with a black flash:
    https://bugzilla.gnome.org/show_bug.cgi?id=771708
    https://bugzilla.redhat.com/show_bug.cgi?id=1370791

    Black background appears briefly before window gets drawn:
    https://gitlab.gnome.org/GNOME/gtk/issues/550

The fix was committed here, three months ago:

    x11: Set a transparent background on windows by default
    "This avoids black flicker on compositing WMs when a window is 
first shown."

https://gitlab.gnome.org/GNOME/gtk/commit/2ce63a86ba689aa41eb47409c889c469497478b0

I noticed the "empty black window" bug when opening GIMP-2.99 editing 
dialogs such as "Curves" and "Gaussian Blur", on both Gentoo (updated a 
couple of week ago) and OpenSUSE Tumbleweed (updated today). Both of 
these OS's use GTK3 version 3.30.

Checking the latest 3.30 tarball from 
http://ftp.gnome.org/pub/gnome/sources/gtk+/3.22/, it appears the fix 
hasn't been incorporated into any actual GTK3 releases.

So I downloaded the latest GTK3 tarball, applied the fix (involves 
adding a couple of extra lines of code to gdk/x11/gdkwindow-x11.c), and 
compiled/installed it in my GIMP-2.99 prefix. This fix does indeed get 
rid of the "black flash" problem.

Best,
Elle