WM_SYSCOMMAND

  This message is like the WM_COMMAND message, but is sent by a window's
system menu.

Parameters
  wParam can be one of the following codes:
SC_CLOSE -  close  the  window
SC_MAXIMIZE -  maximize  the  window
SC_MINIMIZE -  minimize  the  window  (not  implemented  yet)
SC_MOVE -  move  the  window
SC_RESTORE -  restores  the  window  to  its  unmaximized  size
SC_SIZE -  resize  the  window

   The default window proc carries out the predefined actions for each of
   the above requests.
  lParam is not used.