If the knob position changes sufficiently to change a propgclass object's
pga_top attribute, the gadget will send an om_notify message to itself,
which the propgclass dispatcher passes on to the gadgetclass dispatcher
for processing (see the rootclass description of om_notify and om_update
for more information).

The om_notify message will contain two attribute/value pairs, pga_top and
GA_ID.  While the prop gadget's PGA_Top is in a transitory state (while it
is active and the user is moving the prop gadget's knob), the gadget sends
interim om_notify messages.  the interim om_notify messages have the
OPUF_INTERIM flag of the opupdate.opu_flags field set.  when the user
finishes manipulating the gadget (by letting go of the knob), the gadget
sends a final OM_NOTIFY message, which has a cleared OPUF_INTERIM flag.