I'm not sure that it matters all that much what state you're in; whatever the state defaults are, they're "corrected" by boilerplate employment rights assignment contracts.
It matters intensely which state you're in, because quite a few states, such as California, have declared that kind of rights assignment contract unenforceable / illegal.
Don't forget how common it is for companies to sign their staff to nonsense in the hope of scaring them out of their rights through ignorance.
Well in the vernacular, "you can sue for a ham sandwich;" virtually nothing can keep you out of court.
I can sue you, and get you into court, for murdering my pet vampire using the power of forgiveness, if I want to, and am willing to waste the money.
I mean, to your actual point, yes, there are cases where this can come up and be valid, but my understanding of this part of the thread was that we were making comparison to New York state boilerplate contracts that would take ownership of anything produced on side time, and that has been tested and there is a significant body of "oh hell no" in California law.
I think the vernacular is that you can indict a ham sandwich. But yes, my point is just that no matter what US jurisdiction you work in, you probably won't go wrong by adopting the most conservative interpretation of your employment contract.
