https://bugs.launchpad.net/ubuntu/+source/metacity/+bug/878198
Another workaround is to hack the theme itself. For the Ambiance theme edit
$sudo gedit /usr/share/themes/Ambiance/metacity-1/metacity-theme-1.xml
and in the section starting
<frame_geometry name="frame_geometry_normal"
change left_width, right_width and bottom_height from 1 to 3.
I have not tried it on the Radiance theme but I expect the same change would work there. Keep a note of what you have done as if the theme is updated you might have to do it again.