At present PropertyNotify makes calls into window-props for many but not all properties.  It should call in for everything.

My original question about this:
http://mail.gnome.org/archives/metacity-devel-list/2008-August/msg00000.html

Havoc replied ccing metacity-devel-list, but it seems not to have been archived; I'm reproducing it below:

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What "call reload" means is meta_window_reload_property(). If you'll
notice, most of the branches in the if-else just call
meta_window_reload_property(). When *all* the branches just call
meta_window_reload_property(), you can delete the whole if/else and
unconditionally call meta_window_reload_property(). "move to
window-props.h" means "add MetaWindowPropHooks for the property"

The MetaWindowPropHooks are one init func and one reload func, so when
you add them, generally the code on window init that reads the prop is
replaced, and the code on property notify is also replaced.