moljacsharp

I believe MS claims they are "open source 1st" right now.

Everything made by Xamarin was opensourced in 2016-04/2016-05 under MIT license, so it cannot be vendor locked anymore.

Monodevelop/Xamarin.Studio have really simple addin system and writing addin to support anything shouldn't be big problem.

It is a lot easier than Visual Studio Extensibility system.

No way. WPF is too tightly bound to DirectX. Yes there are a lot of requests to do so, but effort is enormous.

Though there are projects that might indicate something (SkiaSharp and VulcanSharp) on mono project github organisation.

Core support (ASP.net Core) was added more than year ago in the form of addin.

Other project types were added recently and I haven't tried them yet, so I would not like to speculate.

https://msdn.microsoft.com/en-us/magazine/mt790182.aspx

Nope.

Most of the UI was rewritten for VS2010 with WPF (XAML, DirectX acceleration), but lower/deeper parts are written in c++ and this makes porting "probelmatic" (MFC, win32.dll, kernel.dll to posix stuff).

MonoDevelop ~ Xamarin.Studio is written in C# (some plugins in F#) and this makes porting easier. That is the reason there was MonoDevelop version on Windows.