This message occurs when the operator presses button 2 of the pointing device twice within a specified time, as detailed in WM_BUTTON1DBLCLK.

param1
     POINTS  ptspointerpos  /*  Pointer position. */

param2
     USHORT  fsHitTestres   /*  Hit-test result. */
     USHORT  fsflags        /*  Keyboard control codes. */