GTK3 "empty black window"/"black flash" bug that affects GIMP-2.99
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
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.
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.