This appears to be a regression from GTK+ 2.x

If you are editing e.g. a GtkCellRendererText, and you dismiss editing just with clicking in another area of the treeview (i.e. moving the focus out of the renderer entry), you won't get any signal.
There's code in GtkCellRendererText to handle this (see gtk_cell_renderer_text_focus_out_event()), but AFAICS what happens is:

- gtk_tree_view_button_press() calls gtk_tree_view_stop_editing (cancel_editing = FALSE) first thing
- that in turns calls gtk_cell_area_stop_editing (cancel_editing = FALSE)
- that calls gtk_cell_renderer_stop_editing (canceled = FALSE)
- gtk_cell_renderer_stop_editing() does this

  if (priv->editing)
    {
      priv->editing = FALSE;
      if (canceled)
	g_signal_emit (cell, cell_renderer_signals[EDITING_CANCELED], 0);
    }

resetting this way its 'editing' property [1].

- later on, from gtk_tree_view_button_press(), the focus-out handler of the renderer is called
- the focus-out handler will call gtk_cell_renderer_stop_editing (canceled = TRUE), which will not emit the signal because priv->editing is FALSE already.

[1] the fact that it does so directly without calling g_object_notify() for the property might be considered as another separate bug.