Code:

procedure TForm1.Memo1MouseMove(Sender: TObject; Shift: TShiftState; X,

  Y: Integer);

begin

  Memo1.SelStart  := LoWord(SendMessage(Memo1.Handle, EM_CHARFROMPOS, 0, MakeLParam(X, Y)));

  Memo1.SelLength := 0;

end;