I'm not sure what the workaround you are mentioning is, but what I noticed was that my template's rendered method was being called twice. I also noticed that when I put $("#combobox").combobox() inside my template's rendered code, it neither rendered in the browser, nor would it render when I then ran that code in the chrome console. It was only when Meteor was not responsible for running the code and I ran it in the chrome console that it rendered.
This suggested to me that the first time my template's rendered method was being called the interface was not ready. Thus, I inserted a counter and made sure that the combobox() method was only called the second time my template's rendered method was called.
I must say this was challenging to debug - too challenging IMHO!