Yeah, kind of "New minor releases may introduce new widgets, or update the implementation of windowing system protocols in the GDK backends, but no additional features or theme changes will be allowed. "
ah cool, ok. So here's what I propose: Keep existing Eto.Gtk3 and deprecate it. New Eto.Gtk3Core (or whatever) will support some specified version of Gtk3 at minimum but use newer apis if available. When Gtk4 comes out, we copy Eto.Gtk3Core to Eto.Gtk4Core and update to 4.0 apis.
The current Eto.Gtk3, being deprecated will not be updated for the next version of Eto and will be removed.