gnm_font_button_init says...

  font_button->priv->show_preview_entry = FALSE;

...but gtkfontchooser.c says it should default to TRUE:

  g_object_interface_install_property
     (iface,
      g_param_spec_boolean ("show-preview-entry",
                          P_("Show preview text entry"),
                          P_("Whether the preview text entry is shown or not"),
                          TRUE,
                          GTK_PARAM_READWRITE));