During download (300+ MB) I opened Thunderbird. I then moved it to a different screen (desktop 1 to desktop 2) The screen selector sub-window did not exit after completing the selection successfully. That sub window remains on the screen (showing the from desktop (1), not showing the desktop 2 selection) as I write this. It is on all desktops. I suppose it is a zombie. 

ad@dad314159:~/Desktop$ ps -elf | grep desktop
0 S dad       1361 23560  0  80   0 -  3221 pipe_w 12:02 pts/3    00:00:00 grep --color=auto desktop
1 S dad       4041  3564  0  80   0 - 77609 poll_s 11:39 ?        00:00:00 desktop.so [kdeinit5] desktop local:/run/user/1000/klauncherXM3569.1.slave-socket local:/run/user/1000/plasmashellTJ3836.1.slave-socket
1 S dad       4071  3564  0  80   0 - 74304 poll_s 11:39 ?        00:00:00 file.so [kdeinit5] file local:/run/user/1000/klauncherXM3569.1.slave-socket local:/run/user/1000/kio_desktophX4041.1.slave-socket
4 S nobody    4346  1315  0  80   0 - 12888 -      11:40 ?        00:00:00 /usr/sbin/dnsmasq --no-resolv --keep-in-foreground --no-hosts --bind-interfaces --pid-file=/var/run/NetworkManager/dnsmasq.pid --listen-address=127.0.1.1 --cache-size=0 --conf-file=/dev/null --proxy-dnssec --enable-dbus=org.freedesktop.NetworkManager.dnsmasq --conf-dir=/etc/NetworkManager/dnsmasq.d
dad@dad314159:~/Desktop$