KDE 4 panel remains "uncomposited" after turning compositing back on

[Guest]

I configured powerdevil to turn off compositing when the battery level is low.

When compositing is disabled this way, the "taskbar" panel will stay opaque (window animations and transparency are OK) after enabling it back on (alt+shift+f12

The only way to get it "composited" back is re-logging or rebooting.

Any hint?