Bit of further investigation shows this issue is due to pup-volume-monitor which is essential for PcManFM and is still running after switching to JWM.davids45 wrote:'Open file' 'Places' panel still cluttered in this series (screenshot) - Bionic Pups are not affected if that helps identify what's happening.
The solution is to remove execute permission from:
/etc/init.d/start-pup-volume-monitor
and reboot