Microsoft has demonstrated the total embracing of open source, as it open-sources the three Windows UX frameworks, namely: Windows Presentation Foundation (WPF), the Windows Forms and Windows UI XAML library (WinUI).
While the open source frameworks will avail developers the ability to contribute to the source code creations, with third-parties input in the debugging of the frameworks to provide timely fixes.
Albeit, the WPF isn't completely open at the moment, only the smaller components are available on GitHub; but Windows Forms and WinUI are completely open and available.
The frameworks will serve as followup to the open source .Net Core 3 framework, which also works with the Universal Windows Platform (UWP) XAML, and existing and new applications can run on .Net Core.
Also, developers can launch and debug projects on WPF and Windows Forms right within Visual Studio 2019 Preview 1, and open .NET Core 3.0 projects in Visual Studio 2017 15.9, although you'll need to enable the previews.
The WPF will serve as a rendering engine to take advantage of the modern graphics hardware, and the UI framework for building desktop client applications, including model, graphics, controls, layout, data binding and security.
Microsoft hopes to scale the growth of .NET by leveraging the open source community, while the developers can help in the debugging and fixing of issues, as well as building private copies of the UI stack.