GtkStatusbar
I’ve been fighting with GtkStatusbar a lot lately, as it’s the only way to get a resize grip to put in the corner of a window. Unfortunately, you’re forced to take a text label widget along with the resize grip.
I did find out that GtkStatusbar can be used as a container; as a GtkHBox, you can pack other widgets in it, but only to the right of the built-in label, between the label and the resize grip.
Looking at the source code, the widget seems very perversely designed to prevent customization—slipping anything to the left of the text label, or even more annoyingly, into the frame with the label, will cause its layout around the grip to fail. Even simple things like turning off the frame border around the label widget are blocked, as it re-sets the frame style on every size_request.
C’mon, Gtk people. Give us a separate grip widget already. :/