I started up my laptop and had no network connection. Neither my wire or wireless connections had an address, since they are both DHCP.
I started zdesktop server. I then started the UI. A dialog box popped up on the screen saying I had to select 'offline mode' from the menu, and the Zimbra display remained blank. (I'm afraid I don't have the exact text of the error).
I did have the loopback interface up.
Does Zimbra Desktop need an active network connection, and consider offline mode simply as not being able to connect to the server? I didn't think so, but I may have missed something.
Next time I'm in the same situation, I'll try firefox instead of prism for a connection.