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

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

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