Steps to repro:
0) In gnome-terminal/src/terminal-window.c:terminal_window_init(), insert this code directly after the "priv->notebook = gtk_notebook_new ();" line:

  {
    GtkWidget *h = gtk_hbox_new (FALSE, 6);
    gtk_box_pack_start (GTK_BOX (h), gtk_button_new_with_label ("hi there!"), FALSE, FALSE, 0);
    gtk_widget_show_all (h);
    gtk_notebook_set_action_widget (GTK_NOTEBOOK (priv->notebook), h, GTK_PACK_START);
  }

1) Compile, run
2) Create a 2nd tab
3) Switch back and forth between the tabs

Results:
a) The action area where the button should be is allocated space, but the button is not shown
b) On each tab switch, the window grows in the horizontal direction!