It's like this:
A bonus you receive for work you actually performed in Massachusetts is fully taxable by Massachusetts regardless of where you live.
If you receive the bonus (or any portion thereof) after you become a resident of California, the amount received will also be taxable by CA. But in that situation, you'd be able to claim a credit on your CA return for the taxes you paid to MA on those dollars - thus avoiding double taxation.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
