I have client 6.0.10 running on 2 different Macs, and they display the same bug. One is a new MacBookPro running 10.9.1 and the other is a MacBook running 10.7.5 which is also the Indigo server.
In the devices list, if I click on the mode pop-up menu for a Venstar thermostat, it is completely blank. I have 3 thermostats, and all 3 show the blank pop-up. The fan pop-up looks fine.