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WM_PAINT

  This message is sent by either an application or the window system when it
wants a window to be redrawn. User-written window procedures should respond
to this message by calling the procedure needed to redraw that window.

Parameters
  wParam is 0 if the entire window should be repainted, or is greater than
   zero if lParam points to an array of RECTs to be repainted.
  lParam is NULL if the entire window should be refreshed, or points to an
   array of RECT structures  which contains the invalid rectangular regions
   to be repainted. The rectangles contain client screen coordinates.