Compiz doesn't paint desktop background when using multiple monitors

[Cefn]

Although Compiz repaint behaviour seems perfect on a single monitor, (my laptop's builtin LVDS), or a mirrored monitor (without extended desktop), you can see the effect of extending the desktop onto an external VGA monitor to my desktop in the attached screenshot.

Foreground window pixels continue to be painted, as far as I can see, but background pixels (or idle window pixels) don't get repainted, making the desktop almost unusable (especially, you don't know when you've closed a window, as it's still painted there).

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: compiz 1:0.9.9~daily13.06.19~13.04-0ubuntu1
ProcVersionSignature: Ubuntu 3.8.0-26.38-generic 3.8.13.2
Uname: Linux 3.8.0-26-generic i686
ApportVersion: 2.9.2-0ubuntu8.1
Architecture: i386
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
Date: Wed Jun 26 10:22:24 2013
MarkForUpload: True
PackageArchitecture: all
SourcePackage: compiz
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.