codice:
  protected
    procedure WndProc(var Message: TMessage); override;
    ...

procedure TMainForm.WndProc(var Message: TMessage);
begin
  begin
    case Message.Msg of
      WM_ACTIVATEAPP: begin
        if Message.WParam= 1 then ... else ...;
      end;
    end;
  end;
  inherited WndProc(Message);
end;