A family user of Lubuntu 18.04 has twice experienced a problem while using Thunderbird where, while dragging an email from the inbox to a local folder, if they mistakenly touch the panel with the cursor, it freezes the entire desktop. They didn't even drop the email on the panel, just touched it.
I confirmed this problem in a virtual clone of their system.
REISUB didn't seem to work. I can ssh in and restart the computer or kill Thunderbird to recover ops.
It only happens if the cursor touches the area of the panel to the left of the rightmost red marker. If it touches the running Thunderbird icon or anything to the right of it, it is OK.
When it first happened, the friendly family IT guy (me) gave counseling "don't do that" but now it's happened again.
I'd like some protection against this happening again if possible or at least a single-user recovery method.
Addition: same problem happens on an 18.04 Ubuntu system with LXDE installed. So looks like a Thunderbird / lxpanel interaction problem.