alt Hacker NewsThe main issues I'm aware of are whether APIs are copyrightable, and if so, whether implementing them qualifies as fair use.
https://en.wikipedia.org/wiki/Google_LLC_v._Oracle_America,_....
(And, of course, Microsoft would also have to consider whether such a lawsuit would have greater benefits than costs. I would like to think that customer goodwill has more than zero value, for example.)
Going back a bit further:
https://en.wikipedia.org/wiki/Sony_Computer_Entertainment%2C...
https://en.wikipedia.org/wiki/Sega_v._Accolade
This fact pattern (reimplementing API functions for emulation or interoperability) tracks even more closely with the Connectix case than Oracle. Google reimplemented a huge swath of the Java API surface so developers could reuse libraries, but actual applications still needed porting, so there's less protection from a fair use perspective; and even then copying APIs was still ruled to be fair use.
I just don't see how Microsoft could contort the facts to achieve a meaningfully different outcome. It doesn't matter if APIs are copyrightable if copying them is fair use for just about any purpose.