The California Residency & Sourcing Technical Manual says this:
Nonresidents
Benefits that are directly related to California employment such as sick leave, bonuses, vacation
pay, and severance pay, are includible as California income. (See Appeal of Edwin O. and Wanda
L. Stevens, 86-SBE-100, May 6, 1986; Appeal of William H. Harmount and Estate of Dorothy E.
Harmount, Deceased, 77-SBE-121, Sept. 28, 1977.)
https://www.ftb.ca.gov/tax-pros/procedures/residency-and-sourcing-technical-manual-disclosure.pdf
Unfortunately, I believe the California FTB may well determine that the bonus must be apportioned according to the number of months worked in each state, on the basis that part of the bonus is "directly related" to California employment.
Since the bonus was actually received as a resident of New York, it is all taxable by New York, but New York will grant an "other state credit" for the tax paid to CA on the portion taxed by both states. This will prevent double taxation of that income.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
