This was not using a xinerama setup - it was on a single laptop, with only the normal screen/display.
The screen was not locked - the example in the screenshots shows a power dialog, but any system dialog which darkens the background when it appears (eg password dialog to access administrative functions) has the same effect. I switched to another desktop using ctrl-alt-F8 etc. -- Screen not properly redrawn on desktop change, if a system dialog is shown https://launchpad.net/bugs/77415 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
