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WM_SETFOCUS

  This message is sent to a window which is receiving the input focus. This
gives the window the opportunity to perform some kind of initialization or
drawing which tells the application that this window now has the input
focus (ie - drawing a double-line border, highlighting the text, etc).

Parameters
  wParam is the handle of the window that just lost input focus, or NULLHWND
   if there was none.
  lParam is not used.