In practice, though, even in CA, I would suggest employees always get something from their company, in writing, disclaiming any ownership of any project they decide to start or work on (in any large capacity), especially if it's a software project and they work for a software company.

Even if you truly believe what you're working on has nothing to do with your employer's line of business, they may disagree, and even if they're wrong, they can make your life difficult and expensive while you try to prove them wrong.

It makes me sad that this is a thing I feel I need to suggest, but I think it's the prudent move, and many companies will even have a fairly painless, well-defined process for doing this.