propgclass intercepts this gadgetclass attribute before passing it on to
gadgetclass.  It does this to make sure the highlighting is not set to
GADGHBOX.  GADGHBOX will be converted to GADGHCOMP.  See the
"intuition gadgets" chapter of the amiga rom kernel reference manual:
Libraries for more information on the types of gadget highlighting.

Other gadgetclass attributes are passed along to the superclass.