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.

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