Comment 14 for bug 769644

Comparing behaviour in Lubuntu with behaviour in Ubuntu that has had LXDE added to it later is probably worth doing. I *think* this issue only happens in the Ubuntu-with-LXDE situation, not if you install Lubuntu.

(Total guess: could lxpanelctl be assuming that the root window it sends messages to is created by some part of LXDE which knows about lxpanel? And perhaps in the "Ubuntu-upgraded" situation, that root window actually belongs to/is created by some other app which has no knowledge of lxpanel, so it does not know what to do with the message, and so discards it?).