Committed today to the GTK+ code-base by Carlos Garnacho was the merging of the "popovers" branch. As described within the code's comments, "[Popovers are] a bubble-like window containing contextual information or options. GtkPopovers can be attached to any widget, and will be displayed within the same window, but on top of all its content."
GTK+ in its 3.11 series ahead of GNOME/GTK+ 3.12 already has brought a new container and widget changes, OS X support improvements, Broadway/HTML5 back-end touch support, iPad/tablet support, and better handling of Wayland. GNOME 3.12 will be officially released in March.