procedure TCalcThread.Execute;
begin
  ......
  PostMessage(MainForm.Handle, WM_MY_NOTIFY, 0, 0);
  ....
end;

procedure TMainForm.WMMyNotify(var AMsg: TMessage);
begin
  UpdateVCL;
  ClearMsgQueue(Handle, WM_MY_NOTIFY);
end;

function ClearMsgQueue(AWnd: HWND; AMsg: Cardinal): TMsg;
var
  LMsg: TMsg;
begin
  Result.message := 0;
  while PeekMessage(LMsg, AWnd, AMsg, AMsg, PM_NOREMOVE) and
        (LMsg.message <> WM_QUIT)
  do begin
    PeekMessage(LMsg, AWnd, AMsg, AMsg, PM_REMOVE);
    Result := LMsg;
  end;
end;