Not possible to drag and drop child-widgets onto GtkMenuButton
Submitted by Tobias Schönberg
It is currently not possible to drag and drop a GtkImage or GtkLabel onto a GtkMenuButton. One can get around this problem by dragging the widget into an empty container and then copying it and right-click pasting it into the GtkMenuButton.
This bug is reproducible in Glade 3.18.3 and 3.19.0.