But they don't need to reimplement it entirely from scratch or as a clean-room implementation because:
1. WPF is now MIT-licensed.
2. Unlike WinForms, the bulk of WPF is managed-code, not a thin-wrapper over Win32.
Therefore anyone can now port WPF to Linux, macOS, etc - and they only need to port the "PresentationCore" part of WPF (i.e. the native parts that expose Direct3D drawing surfaces to WPF).
