With the GNOME 3.16 series, Mutter uses GTK+ for drawing all window decorations regardless of using client-side decorations, and there's also many other GTK+ improvements.

For those wishing to learn more about the GNOME 3.16 improvements with regards to drawing window decorations, Florian Müllner has written a new blog post showing off the window decoration changes with this new release.

GNOME 3.16 is expected for release in March and besides the GTK+ changes there's also many other improvements to this desktop environment.