> California has a very broad definition of what being a "resident" is, so you can be a full-time resident of multiple states despite only having a single domicile and mailing address.
No, it doesn't.
> so you can be a full-time resident of multiple states despite only having a single domicile and mailing address.
You cannot (well, not for California, though perhaps for some combination of other states), though you could be a tax resident of multiple states in the same tax year by having a series of domiciles during the year that each were not intended to be transitory when established.
(If you move directly from California to a foreign country, but retain citizenship and return to California for more than 45 days in a future tax year, you can also be a California tax resident with a domicile in a foreign country, but because it is keyed on being a California resident immediately before departing (and not just for any part of the tax year), that wouldn't be a backdoor way for multi-US-state tax residency unless the other state had a broader rule.
> States generally either tax the global income of their residents (e.g. California), or they tax income earned within the state (e.g. Connecticut).
California taxes global income of residents and income earned in the state by nonresidents; this is, AFAIK, the majority rule for states that have income taxes.