WM_NCHITTEST is for hit-testing, and hit-testing can happen for reasons other than the mouse being over your window

The WM_NC­HIT­TEST message is sent to your window in order determine what part of the window corresponds to a particular point. The most common reason for this is that the mouse is over your window.The default WM_SET­CURSOR handler uses the result of WM_NC­HIT­TEST to figure out what type of cursor to show. for example, if you return HT­LEFT, then Def­Window­Proc will show the IDC_SIZE­WE cursor.