I am encountering this bug as well. I am able to duplicate it under xfce4 as well as awesome. When running cssh with either the -G or -g flag and switching to the virtual workspace that it resides on the cssh input window completely disappears for around a second and then reappears. If you change away from this virtual workspace before it "reappears" it will come up on the workspace you just switched to thus causing the extremely frustrating behavior.
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