GTK+ 3.3.6 does feature changes like widget improvements, better theming support, refactord completion coide for GtkFileChooser, and other work, but catching the Phoronix interest the most is that the Wayland GDK back-end has been updated to the current Wayland API. GNOME's tool-kit is updated just days after the Clutter Wayland support was updated.
Guess what? The Wayland GTK+ updates come courtesy of Rob Bradford, the same Intel OSTC developer responsible for the recent Wayland back-end updates to Clutter/Cogl. He's also the one that's been working on porting portions of Google's Chrome/Chromium OS to this next-generation display server. Read Quickly Emerging Wayland Support For Chrome OS.
The Wayland-related commits to GTK+ can be found from the web-based Git viewer.
The email release announcement shares other details about the GTK+ 3.3.6 release.