GTK for long time was considering as the main weakness of Gnome. These days, GTK has been transformed to one of the most slick UI Toolkits around. Many new widgets have landed in the last year, and theming has received major improvements. While there are much left to be done, the overall outcome already looks pretty good.
The above figure is an experimental design for GTK List Widget, made 10 days ago by Allan Day. The hot part here is the Popover Menu rather the List. Popovers are core items of all modern UIs, but they were missing from GTK.
The design roadmap of GTK reminds a lot of Bootstrap Twitter Framework, and the GTK Popovers are not an exception.
They (Gnome devs) are working on them and hopefully Popovers will arrive in Gnome Three-Point-Twelve. No guarantees.
You can try them your selves from Gtk Popovers Branch.
GtkPopover is a bubble-like context window, primarily meant to provide context-dependent information or options. Popovers are attached to a widget, passed at construction time on gtk_popover_new() or updated afterwards through gtk_popover_set_relative_to(), by default they will point to the whole widget area, although this behavior can be changed through gtk_popover_set_pointing_to().
The position of a popover relative to the widget it is attached to can also be changed through gtk_popover_set_position().
By default, no grabs are performed on #GtkPopovers, if a modal behavior is desired, a GTK+ grab can be added with gtk_grab_add().