The spin button update policy determines whether the spin button displays values even if they are outside the bounds of its adjustment. See the function gtk-spin-button-update-policy.

(define-g-enum "GtkSpinButtonUpdatePolicy" gtk-spin-button-update-policy
  (:export t
   :type-initializer "gtk_spin_button_update_policy_get_type")
  (:always 0)
  (:if-valid 1))  

:always

When refreshing the spin button, the value is always displayed

:if-valid

When refreshing the spin button, the value is only displayed if it is valid within the bounds of the spin button's adjustment.