I was looking at what it would take to get a cross-platform client GUI library built on top of .NET Core, and I believe the path of least resistance would be to port Mono's WinForms and System.Drawing implementations.
Their WinForms draws controls using System.Drawing and has OS-specific drivers for Windowing and Eventing. System.Drawing pushes all the platform differences into a native library that it P/Invokes.
I know, I know, cross-platform GUIs are ugly, but like I said, I think this is the path of least resistance to getting client apps working on top of .NET Core. Large parts of the implementation may also have to be removed or changed to fit the smaller .NET Core profile.
Back to my original question: Is anyone already working on this or something like it?