The container control sends the WM_CONTROL (in Container Controls) message with the CN_COLLAPSETREE notification code to its owner whenever the container collapses a parent item in the tree view.

param1
     USHORT       id               /*  Container control ID. */
     USHORT       CN_COLLAPSETREE  /*  Notification code. */

param2
     PRECORDCORE  pRecord          /*  Pointer to the record that was collapsed. */