xfwm4 killed with SIGTRAP from glib in clientSendConfigureNotify client.c:699 calling XSendEvent().

Summary

Xubuntu 20.04, up-to-date, no heavy configuration tweaking or special software installed, no PPA (initially). Machine seems sane, no mysterious crashes.

xfwm4 crashes (and is automatically restarted) about once a week. (By the way, after xfwm4 restart, all windows get moved to the right and bottom by the thickness of left and top decorations in the style used.)
I logged in on /dev/tty1 and there started gdb which ran fxwm4 --replace.
Installed Ubuntu PPA for dbgsym packages and xfwm4-dbgsym.
This time, crash happened after machine (a laptop) woke from suspend-to-ram. Other crash conditions exist.
X session was reachable but could not focus any window, so I guessed xfwm4 was frozen.
Indeed on /dev/tty1 gdb was available. Captured stack trace and core dump.
Stack trace is attached: gdb_xfwm4_02_bt.txt
Core dump available, just ask.