I'm always so paranoid of this at any of my employers so I go through great lengths ensure everything is separated. It's a lot more work in terms of managing your stuff, but can simplify situations like these. I always keep my work and my personal projects completely separated. Separate machines, separate repos, separate accounts and I don't do personal stuff at work. My goal is to have a clear separation so if either of us decide that our contract isn't working; it's as frictionless at possible. While I applaud the fight for what's right, there is no doubt that a legal disagreement will take time away from building the product that your users want.