(See bug 72101 against the GNOME HIG for a general discussion about tab
behaviour.)

Some application use colors or icons to display the state of a tabbed page.
It might be a worthwhile addition to Gtk+ to add the ability to do this easily.

API draft:

  enum _GtkNotebookState {
      GTK_NOTEBOOK_STATE_NORMAL,
      GTK_NOTEBOOK_STATE_LOADING,
      GTK_NOTEBOOK_STATE_CHANGED,
      GTK_NOTEBOOK_STATE_ATTENTION,
      GTK_NOTEBOOK_STATE_UNSAVED
  };
  typedef enum _GtkNotebookState GtkNotebookState;

  void gtk_notebook_tab_set_state(GtkNotebook *notebook, gint position,
GtkNotebookState state);
  GtkNotebookState gtk_notebook_tab_get_state(GtkNotebook *notebook, gint
position);

This could draw an appropriate icon at the tab labels. The advantage of
such an API is the ability to abstract the implementation and streamline
applications.

An alternative or an addition might be a convenience API like:

  gtk_notebook_tab_set_icon(GtkNotebook *notebook, gint position, GtkWidget
*icon);
  GtkWidget *gtk_notebook_tab_get_icon(GtkNotebook *notebook, gint position);

Please note that all this is fairly easily implementable in applications,
too, so if you don't consider this a worthwhile addition to Gtk+, don't
hesitate to close this bug. I will prepare an addition to the HIG that will
 recommend application authors to use a certain style when doing things
like these described in this proposal. This addition could be removed from
the HIG if this proposal was implemented.