Holy cow, I wrote a book!
Leo Davidson observes that
a hit-test code is defined for
HTOBJECT, but it is not documented, and wonders
#define HTOBJECT 19
The HTOBJECT is another one of those features
that never got implemented.
The code does nothing and nobody uses it.
It was added back in Windows 95 for reasons lost to the
mists of time,
but when the reason for adding it vanished (maybe a feature got cut),
it was too late to remove it from the header file because that would
require renumbering HTCLOSE and HTHELP,
two values which were in widespread use already.
So the value just stayed in the header file,
taking up space but accomplishing nothing.