function ClientWindowProc(wnd: HWND; msg: cardinal; wparam, lparam: integer): integer; stdcall;
var
 P: pointer;
 Style: dword;
 ExStyle: dword;

begin
 P := Pointer(GetWindowLong(wnd, GWL_USERDATA));

 case msg of
 WM_NCCALCSIZE:
 begin
   Style := GetWindowLong(wnd, GWL_STYLE);
   ExStyle := GetWindowLong(wnd, GWL_EXSTYLE);

   //Убираем скроллы MDI-окна
   if (Style and (WS_HSCROLL or WS_VSCROLL)) <> 0 then
   begin
     Style := Style and not (WS_HSCROLL or WS_VSCROLL);
     SetWindowLong(wnd, GWL_STYLE, Style);
   end;

   //Убираем выступ в клиентской области MDI-окна
   if (ExStyle and (WS_EX_CLIENTEDGE)) <> 0 then
   begin
     ExStyle := (ExStyle and not WS_EX_CLIENTEDGE);
     SetWindowLong(wnd, GWL_EXSTYLE, ExStyle);
   end;

   Result := S_OK;
 end;//WM_NCCALCSIZE
 end;//case

 Result := CallWindowProc(P, Wnd, Msg, WParam, LParam);
end;

//................................................................................ ......................................

procedure InitClientProc(ClientHandle: integer);
begin
 if (ClientHandle <> 0) then
 begin
   //Данные пользователя могут быть заняты
   if GetWindowLongPtr(ClientHandle, GWL_USERDATA) = 0 then
   begin
     SetWindowLongPtr(ClientHandle, GWL_USERDATA,
       SetWindowLongPtr(ClientHandle, GWL_WNDPROC, NativeUInt(@ClientWindowProc)));
   end;
 end;
end;