I created a small GTK+ widget based on GtkWindow which I called GtkDrop.
I once wanted to create a popup calendar for gnome-schedule and this is basically that code put into a widget (and ported to C).
It is intended to be a 'popup'/'dropdown' container where you can add any widget and it will behave similar to menus or comboboxes. And you will be able to use the widget inside it, ie: write in a GtkEntry or press a GtkButton.
In case you want to try it out download it, the test program is included.