At least in the UK I was under the impression it's pretty illegal to try to claim anything employees do out of work time as company property

I know that in my particular field (offensive cybersecurity) many, if not most, places that I’ve heard of, will carve out allowances for personal projects to remain yours. Some companies will even be fine with you setting aside a portion of your time each week to dedicate to developing and maintaining your own open source community tooling or contributing to projects you use regularly, without that whole “your ideas are our IP” thing. With that said, these are all smaller shops that are competing to hire hyper-specialized talent in an industry that until recently wasn’t as overrun with people as the development space is, so maybe none of this is applicable to a place like MS, I don’t really know.