On brainwash's suggestion (IRC #xubuntu-devel) I disabled UPower's Lid handling with /etc/UPower/UPower.conf:
IgnoreLid=true
After six suspend-resume cycles this does seem to have prevented the issue.
*However* I then noticed that in the x-p-m dialog, General tab, the options to control the Lid Close policy are completely missing - which isn't really a solution.
This just seems to confirm the bug here is in the interactions of x-p-m/light-locker and logind.
On brainwash's suggestion (IRC #xubuntu-devel) I disabled UPower's Lid handling with /etc/UPower/ UPower. conf:
IgnoreLid=true
After six suspend-resume cycles this does seem to have prevented the issue.
*However* I then noticed that in the x-p-m dialog, General tab, the options to control the Lid Close policy are completely missing - which isn't really a solution.
This just seems to confirm the bug here is in the interactions of x-p-m/light-locker and logind.