IANAL but UI design is not copyrightable. Also let them send you a C&D and then you can get the internet to send MSFT some online hate, which would be a much mo

IANAL but UI design is not copyrightable. Also let them send you a C&D and then you can get the internet to send MSFT some online hate, which would be a much more effective fuck-you than keeping your project hidden out of fear of their corporate legal team (which is just what they want, so they can maintain tacit control over non-existent IP and not have to do any work for it).