Currently, the ToolInvoker code passes the button child in such a
way that the WidgetInvoker queues only the child's draw. In previous
Gtk+ versions, this magically resulted in redrawing the parent as well.
However, Gtk+ 3.20 is probably trying to save power and not randomly
redraw widgets. To fix this, we must queue the redraw of the widget
that we want to redraw explicitly.