GtkFileChooserWidget is not a subclass of GtkBox
In gtk_file_chooser_widget_get_default_size()
(https://gitlab.gnome.org/GNOME/gtk/blob/ba6c5ef2017ad957e914d6fa964a1dc1d566b2c2/gtk/gtkfilechooserwidget.c#L6095), a GtkFileChooserWidget
is dynamically cast to GTK_BOX
, with the resulting
GLib-GObject-WARNING **: 19:12:17.310: invalid cast from 'GtkFileChooserWidget' to 'GtkBox'