Comment 8 for bug 1002792

Cimi (thanks!) suggests searching for "transition" in gtk-widgets.css and make some tests with Precise-stock theme.
He says it's probably a bug in gtk regarding that thing.

For comparison, here's the relevant part in Quantal's theme:
$ cat /usr/share/themes/Ambiance/gtk-3.0/gtk-widgets.css | grep transition
     transition: 750ms linear loop;