Using gtk_box_new() instead of gtk_vbox_new() changes the way child
widgets are packed by default via gtk_container_add().  gtk_vbox_new()
expands them by default, gtk_box_new() does not, and the preview pane
was being packed with gtk_container_add().

Packing widgets into a GtkBox with gtk_container_add() is usually the
wrong way.  Use gtk_box_pack_start() instead, with expand=TRUE.