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gtk_widget_destroy() removes widgets from their container. However _internal_ widgets must be unref'ed using gtk_widget_unparent() instead. This is symmetric with the fact that these widgets were ref'ed by direct call to gtk_widget_set_parent(). It's also the method that was used in gtk_headerbar_destroy(). https://bugzilla.gnome.org/show_bug.cgi?id=772859
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