Same issue on Ubuntu 12.04 with an nVidia card. A fix would be nice, until then here's the workaround I use:
Cinnamon Settings > Themes > Other Settings
Switch the GTK+ Theme to anything and back to your regular one
(Menu item names may differ, I'm not using the English interface.)
I referenced this thread on github to make sure the devs see the issue:
https://github.com/linuxmint/Cinnamon/issues/688