Open Source: WPF, WinForms, WinUI

Microsoft Open Sources WPF, WinForms, and WinUI @ InfoQ

"at Microsoft Connect 2018, the company announced the released of the first preview of .NET Core 3.0. Contained within that is a bigger announcement: the company is releasing WPF, Windows Forms (WinForms), and WinUI as open source projects on GitHub. They have all been released under the MIT license, giving developers wide latitude to use them in their own applications."

Announcing Open Source of WPF, Windows Forms, and WinUI at Microsoft Connect(); 2018 - Windows Developer BlogWindows Developer Blog

GitHub: WPF, Windows Forms, and WinUI. (Backward-compatible versions of Windows UI features including UWP XAML controls, and Fluent styles and materials.)

Open Source versions are based on .NET Core, not on original Windows implementations.
.NET Core is portable on many platforms and high-performance, so this is an interesting option for portable UI.

Microsoft Connect(); 2018 | Channel 9