I don't think CA is THAT much different in this regard. They can still claim ownership of inventions and IP for things done at home on your own time and equipment if it relates to the employer’s business or anticipated research or development (emphasis mine), which, for many large companies encompasses every project you could think of.
EDIT: Odd voting on this one. It's literally in my employment agreement. You might want to check yours!
Without making any argument about what is legal in California, I will note that it is not particularly valid to argue for “it is California law” on the basis of “its what my employer claims in the employment agreement”. Employers are notorious for claiming things beyond what the law allows, knowing that the resulting effect on employee behavior is nearly as good as if the claim was actually enforceable.
AFAIU, at least as it regards federal copyright law "scope of employment" is usually interpreted very liberally in favor of the employer. I can't speak to California case law, but I suspect that the tolerance Silicon Valley has historically shown to employees walking away w/ their "personal time" projects is more a reflection of the normative business culture than a legal restraint. I wouldn't expect IBM to walk away from WhatsApp the same way Yahoo did if one of their Almaden employees had created it.
Twice I've asked an employer for an amendment to the assignment contract to say that only works developed on my own time "with the primary purpose of benefiting the company" were assignable. Though, the second time I was told to f' off; to take it or leave it.
Now, California's legal hostility toward non-compete agreements is clear. It's not just because of the statutory law, but also because of precedent. Other jurisdictions could and have interpreted similar language differently (including other states interpreting California statutes, IIRC). That hostility is usually what people refer to, or would mean to refer to it they understood the nuances.
As you mentioned it I was to reiterate: you can negotiate on anything. Every time I had a non-compete clause in the contract (fortunately not that ubiquitous in Germany) I negotiated with the company to take it out & every time they eventually agreed (once I had to reached a compromise that I should inform them of such projects but the IP remains mine).
A lot of the time you'll hear "oh this is a standard contract, everyone signs it & we don't make exceptions". If they want you bad enough they will make an exception.
If they want you bad enough they will make an exception.
Most people don't have the luxury of walking away from an employer that won't make an exception with the confidence that they can find an employer who will.
EDIT: Odd voting on this one. It's literally in my employment agreement. You might want to check yours!
EDIT: OK, let's look at CA labor code, specifically 2870 (a) (1): https://leginfo.legislature.ca.gov/faces/codes_displaySectio...