[piaste]

That thought experiment sounds off to me. What does "paying to invent" mean, exactly?

Presumably the employer wouldn't be happy if the designer simply had a bunch of nice design ideas and kept it in his or her head.

Instead, the designer should at some point write those ideas down and hand them over as a design document to the rest of the company. That process can provably happen during work hours, and the contents of the design document should be subject to the employment contract - since that's what the employer is paying for.

The document could, for example, have a boilerplate legal heading with "I declare all the contents herein are the sole IP of $company" or some such. Or the contract could specify that any document produced during work hours should satisfy such requirements. That would seem a safer legal protection than simply asserting the right to what the designer came up with on the previous day.

After all, if the concern is that the employee could have invented the game in the evening and then submitted it during the day - well, he or she could just as easily have invented it before their employment began and submitted it afterwards, no?