It wasn’t a fork. It was one and the same. Xamarin was started by Miguel De Icaza [1], who also created Midnight Commander, GNOME, and Mono, the .Net runtime for Non-MS platforms. Xamarin was bought by Microsoft. Mono Develop became Xamarin IDE became Visual Studio for Mac.

I know this because I contributed. Along with working alongside the grandfather of MonoGame. In 2010, .Net was in a weird place.

[1] https://en.wikipedia.org/wiki/Miguel_de_Icaza

> VS for mac just looked like a fork of the Xamarin IDE I used to use on Linux w Mono

I second that. And VS for Mac was painfully slow. (I tried it years ago, not sure how it is nowadays)