kdiff3 on xfce 4.14.5, window decorations missing?

[Mark Mielke]

This was first encountered on Oracle Linux 8.2 + EPEL with xfwm4
4.14.2 + kdiff3 1.8.4 + TigerVNC 1.10.1.

Sometimes the kdiff3 window decorations would be missing. I was able
to reproduce this on Fedora 32. 50% of the time I can reproduce with:

1) Inside a TigerVNC + XFCE session
2) Launch "kdiff3" with no arguments from a terminal window.
3) If the window + open file dialog comes up ok, first close the open
file dialog, then maximize the kdiff3 window and try again to launch
"kdiff3" with no arguments from a terminal window.
4) The main window (QMainWindow, since this is a KDE / Qt app?) is
missing decorations. The dialog window looks fine.

I found a few fixes in 4.14.x related to windows decorations,
including a gsetting / glib corruption issue. But, the same problem
happens with xfwm4 4.14.5 on both Oracle Linux 8.2 and Fedora 32.

Any ideas of where to start looking here? I don't mind trying patches
or even writing patches - but I need help knowing where to start. All
the known issues seem to be addressed in 4.14.5, and don't seem to
have fixed the problem.

Thanks!

-- 
Mark Mielke <mark.mielke at gmail.com>