Happening the past few days in Cambridge, Massachusetts was a GTK+ hackfest where a variety of improvements were tackled. Among the work is GTK+ 3.10 making the file chooser side-bar be a public widget (to already be used by Nautilus), proper composite containers support, baseline alignment work, and new widgets.
Among the new widgets being worked on is a GtkHeaderBar as part of the client-side decorations efforts, GtkStack and GtkStackSwitcher have been committed to master, and GtkRevealer.
Also talked about were working on an EggListBox, a new row container widget, scalability improvements, GtkSettings improvements, more client-side decorations improvements, a simplified GDK drawing scrolling model, and a speed-up to GObject instantiation.
More details on these recent GTK+ 3.10 advancements can be found in this blog post by Red Hat's Mathias Clasen. The big feature already in for GTK+ 3.10 is the Wayland CSD support.