Leo Davidson observes that a hit-test code is defined for HTOBJECT, but it is not documented, and wonders what's up.

#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.