Widget.registerWindow

Registers a #GdkWindow with the widget and sets it up so that the widget receives events for it. Call [gtk.widget.Widget.unregisterWindow] when destroying the window.

Before 3.8 you needed to call [gdk.window.Window.setUserData] directly to set this up. This is now deprecated and you should use [gtk.widget.Widget.registerWindow] instead. Old code will keep working as is, although some new features like transparency might not work perfectly.