With GTK+ 2.99.0, the support for multiple GDK back-ends in a single library is now working. This has already been talked about on Phoronix as it now makes it possible to have support for both Wayland and X11 in a single library or also support for Quartz and X11, etc. This is a huge usability and adoption improvement especially for those two scenarios. The Mac OS X Quartz back-end has also been ported to the new GDK API with this release.
Other changes in GTK+ 2.99.0 include the CUPS print back-end sending print jobs directly as PDFs on supported CUPS setups, many bug-fixes, and work on deprecating and removing certain parts of the GTK/GDK API continue.
See the release announcement for more information.