You are correct, you don't have to pay California tax on the gains you realized when you were not a resident of California. Buying a stock (or other investment) while living in California does not make that investment California sourced. The question is referring to things like employer stock plans that were issued while living and/or working in California, and the leaving the state before they vest or are exercised, triggering a taxable event. California taxes such employer stock, as it was earned/awarded while living/working there. You don't have to worry about thing you bought while living there.
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