Of course you shouldn't think about your portfolio at all during work. I think that's not professional.
Why not? Just because you're thinking something while at work then it isn't professional. As long as it doesn't affect the work load then it's fine.
I agree. It is about the output or the result of your work. As long as you can deliver on what the company demanded then there is nothing wrong to multi-task. It is a good news to know that there are people who are working on their dreams and multi-tasking while they are at work and after from work because that is the true dedication that a success requires.