I haven't tried this latest gthumb (my system is not handy), but I see in the Clutter README that it depends on the XComposite extension. That can be a problem on multi-head systems.
People with only two displays attached to the same card can get around the problems by using things like NVIDIA's Twinview, which tricks the X server into thinking you only have one display. When you add a second card to get three displays it all falls apart. Twinview doesn't work across cards, so you have to enable Xinerama (or live with not being able to move windows between screens). Enabling Xinerama causes XComposite to be disabled because they're not compatible. Xinerama is supposedly deprecated and replaced by XRandR -- which is compatible with XComposite -- but XRandR also doesn't work with multiple cards and and that capability has been repeatedly delayed for something like 3 years now.