The TTHITTESTINFO structure contains information that a tooltip control uses to determine whether a point is in the bounding rectangle of the specified tool. If the point is in the rectangle, the structure receives information about the tool.
typedef struct _TT_HITTESTINFO { // tthti 
    HWND hwnd; 
    POINT pt; 
    TOOLINFO ti; 
} TTHITTESTINFO, FAR * LPHITTESTINFO; 
 
This structure is used with the TTM_HITTEST message.