This seems to me like a bug I've seen here many times (Bug #131526 and many, many duplicates). Are you using a gtk-theme that is not from an ubuntu-package (e.g. from gnome-look.org)? If so, please try out clearlooks or human theme.
This seems to me like a bug I've seen here many times (Bug #131526 and many, many duplicates). Are you using a gtk-theme that is not from an ubuntu-package (e.g. from gnome-look.org)? If so, please try out clearlooks or human theme.