@McCartF asked:
"Can I file a CA 540NR return asking for a total refund of all of my withholding?"
Yes. If you were a full-year non-resident of California, and you never physically worked within California, your W-2 income from that work is not taxable by California. If your employer in that situation mistakenly withheld CA taxes from your pay, your proper recourse would be to file a non-resident CA tax return, reporting the CA withholdings but allocating zero income to CA.
That will result in CA refunding the incorrectly withheld taxes to you.
If your state of residence during that year was MA, your income would be 100% taxable by MA regardless of your employer's location. If your employer in that situation will not withhold MA taxes for you, you must make quarterly estimated tax payments to MA in order to avoid any penalties.
Employers do not set the rules for state income taxation; the states themselves do.
Note: My answer applies only to W-2 employees. As @Opus 17 pointed out, CA's rules for independent contractors - those who receive a 1099-NEC or 1099-MISC - are different.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
