You can have a Web UI in desktop app. Just run the app with embedded kestrel web server and make all UI in HTML. And it will look pretty good an all platforms.

CatLight is one of the apps that do that - https://catlight.io

That's Electron, .NET Core (?) and Squirrel, all working together. Looks pretty cool, just pity the people on <20 megabit connections - these apps are getting pretty big.

Universal Windows Platform ".NET Native" stack is aligned with (built upon/merging towards) .NET Core. It's XAML-based like WPF and Silverlight.

ETA: Other than not cross-platform. Other option is Xamarin.Forms and would be useful to see more Xamarin.Forms targets.

yea, xamarin.forms targets UWP.

and looks like there's a forms mac branch being somewhat worked on: https://github.com/xamarin/Xamarin.Forms/tree/mac