> They could well try to claim stake or ownership on your company...

Note that not only is this not legal in California (but is in others -- Texas is particuarly notorios) but any employment agreement in California must specifically include a copy of the relevant section of law so that the employee is a aware of their rights. But don't use any of the employer's property (e.g. computer, IP) or do it when you are supposedly working: in that case the law does not apply and the employer does get the rights.

If they hire as a contractor, then it's easy to say no to these clauses. If they hire through a local subsidiary then these clauses are against the law in much of the Europe.

How the hell can they justify this habit of telling a developer what they can and can't work on in their own time, or making any claim to it afterwards just because someone worked at company X during the same time they made side project Y? The whole tendency seems grossly onerous and unjustifiable to me.