Comment by megapoliss

https://github.com/picoe/Eto

  - OS X: MonoMac or net6.0-
  - Linux: GTK+ 
  - Windows: Windows Forms (using GDI or Direct2D) or WPF

Gtk

  - Gtk3: https://github.com/GtkSharp/GtkSharp
  - Gtk4: https://github.com/gircore/gir.core