Comment # 8 on bug 1186365 from Andrei Borzenkov

Most likely fixed with
https://github.com/xfce-mirror/xfwm4/commit/ada0d22fd7124f24deabda8b262e9a4c44d88018.
Xfwm4 4.18.0 does not have this issue. Commit itself is pretty trivial and
should be easily backportable.