cancelButton

This property determines what happens when the Escape key is pressed while the alert is shown.

If this property holds the index of a button in property@Gtk.AlertDialog:buttons, then pressing Escape is treated as if that button was pressed. If it is -1 or not a valid index for the buttons array, then an error is returned.

If buttons is NULL, then the automatically created 'Close' button is treated as both cancel and default button, so 0 is returned.