State tax filing

@TomD8  Hope you've been well! I agree with you here, mostly.

California is a physical presence state as it relates to W-2 employees. You are only taxed on days you worked for your employer within the State of California. It doesn't matter if your primary office and boss are located in California, it is not one of the five states with a telecommuting/convenience law (Arkansas, Delaware, Nebraska, New York, and Pennsylvania are the only ones). Connecticut is technically a physical presence state, but as of 1/1/2019 they will honor tax credits for residents subjected to the convenience rule in other states. 

HOWEVER, Massachusetts enacted temporary sourcing rules during the pandemic:

https://www.mass.gov/technical-information-release/tir-20-15-revised-guidance-on-the-massachusetts-t... 

Pursuant to the  regulation as extended, until 90 days after the state of emergency in Massachusetts is lifted, all compensation received for services performed by a non-resident who, immediately prior to the Massachusetts COVID-19 state of emergency was an employee engaged in performing such services in Massachusetts, and who began performing services from a location outside Massachusetts due to a Pandemic-Related Circumstance, will continue to be treated as Massachusetts source income subject to personal income tax under M.G.L. c. 62 and personal income tax withholding.

The regulation also provides rules for non-resident employees who, prior to the Massachusetts COVID-19 state of emergency, determined their Massachusetts source income by apportioning their days spent in Massachusetts in accordance with 830 CMR 62.5A.1(5)(a).  Pursuant to 830 CMR 62.5A.3(3)(b), such non-resident telecommuting employees must determine the portion of their wages that constitutes Massachusetts source income during the Massachusetts COVID-19 state of emergency based on either (1) the percentage of their work days spent in Massachusetts during the period January 1 through February 29, 2020 as determined under 830 CMR 62.5A.1(5)(a), or (2) if they worked for the same employer in 2019, the apportionment percentage properly used to determine the portion of their wages from that employer that constituted Massachusetts source income as reported on their 2019 Massachusetts personal income tax return.

This regulation was in effect until September 13, 2021. Meaning its possible that CA companies, under MA temporary regulations, could have treated any income earned from 1/1/2021-9/13/2021 as CA based income if that was where the employer worked pre-pandemic.