Would be interested to hear about specific court decisions!
In general, this topic is not quite so clear, even in Germany. For a contrary argumentation, have a look at: https://www.lieb-online.com/files/luxe/publikationen/Urheber... (covers both Urheberrecht and Patentrecht, 15 pages, argues seemingly mostly in favour of the employer, but that does not mean they are wrong)
Keywords to search for, if you don't have the time now to read it as a whole:
- Freizeitwerk / freiwilliges Werk (it's a difference! but just because you do something in your freetime it is not necessarily a Freizeitwerk, in case your job is to produce such works and it could be of use to your employer, this is arguably not the case)
- Beweislast (just because you say or mean it to be unrelated to work, does not automatically mean it is -- side note: the bigger your employer, the less you can know about what is in their interest or not)
- Anbietungspflicht (describes the case using work ressources / work time)
- Pflicht zur Anbietung (for the free time stuff which is not "totally unrelated to the interests of your employer" -- so "automatic transition of usage rights via contract" is indeed suspicious and likely to be undermined in court, but they have a say if they want it)
Sounds somewhat like slavery indeed. ;)