WM_BORDER

  This message is sent to a window just before the window manager draws the
window's border. If the user wants to draw his own border, then the window
procedure should handle this message and return TRUE. If you want to let
the window manager draw the border, then the window procedure must ignore
this mesage and return FALSE.

Parameters
  wParam is the type of border that the system wants to draw. It is 0 for a
   single-line border and 1 for a double-line border.
  lParam is not used.