Comment 8 for bug 512679

I did some more testing and discovered a strange behaviour. The problem seems to be related to the "scale display" setting.
It is set to "only when fullscreen" by default.
If I set it to "never" before switching to fullscreen, the problem is stil there

If I set it to "always" before switching to fullscreen, fullscreen mode works.
Interestingly, if I leave fullscreen after entering it on setting "always", I can change "scale display" to "never" or "only when fullscreen" and switching to fullscreen works now, too. (Until the next time I start virt-manager).

It seems there's a state somewhere, either in virt-manager or in metacity which is only set when using the "always" mode.
I hope that helps.

So now at least I've got an easy workaround.