not sure if that fits into this sub-forum, yet I haven't found a better place to ask.

A lot of employment contracts state that all IP (that implies source code, art assets etc.) created during the time of employment belongs to the company, thus even your spare time efforts. Some contracts state that you have to ask for permission to be allowed to even just start working on something in spare time. (means: no weekend game jams etc.).

I wonder, how is the legal state of this in various countries? I know there are some limitations to it in California, some countries even render those parts of contracts invalid, yet it's hard to google for it (or I just use wrong term?). Someone like Notch created his Minecraft as spare time project and it seems his employer never claimed rights for it. that makes me all wonder what's the state of spare time project IP in US states and European countries.

I'd be fully fine with just links to read the specific laws.

You don't "Have" to sign a contract that an employer puts in front of you. I have negotiated my contracts in the past and just told them to remove this clause and they have been happy to do so.

I thought gamedev.net has people from all over the world and someone could tell/referenz to their law.

OK in the UK an employer can claim the copywrite to any work produced while in the course of your employment. What constitutes "in the course of your employment" is not defined. There have been cases in the past where the court has found that an employee who has developed software in their own time outside of work that the work is still owned by their employer because the type of software produced is the same as they would be doing in their day job and they could not proove that the isperation they had to write their own software didn't happen whilst at work. There are also similar cases that have found in favor of the employee.

The law is deliberatly vague and the only person who can win such a case is the one with the best lawyer. The only reason to know for sure is to specifically have it written in your contract that you get to own all respective IP.