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//...
  private
    PRichEdWndProc, POldWndProc: Pointer;
    procedure RichEdWndProc(var Msg: TMessage);
//...


procedure TForm1.FormCreate(Sender: TObject);
begin
  PRichEdWndProc := MakeObjectInstance(RichEdWndProc);
  POldWndProc    := Pointer(SetWindowLong(RichEdit1.Handle, GWL_WNDPROC,
    Integer(PRichEdWndProc)));
end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  if Assigned(PRichEdWndProc) then
  begin
    SetWindowLong(RichEdit1.Handle, GWL_WNDPROC, Integer(POldWndProc));
    FreeObjectInstance(PRichEdWndProc);
  end;
end;


procedure TForm1.RichEdWndProc(var Msg: TMessage);
begin
  Msg.Result := CallWindowProc(POldWndProc, RichEdit1.Handle, Msg.Msg,
    Msg.wParam, Msg.lParam);

  if (Msg.Msg = WM_VSCROLL) and (LOWORD(Msg.wParam) = SB_THUMBTRACK) then
  begin
    Label1.Caption := 'Pos is ' + IntToStr(HIWORD(Msg.wParam));
    RichEdit2.Perform(Msg.Msg, Msg.wParam, Msg.lParam);
    SetScrollPos(RichEdit2.Handle, SB_VERT, HIWORD(Msg.wParam), True);
  end;
end;