interface

{...}

type
  TFormThread = class (TThread)
  private
    FApplication : TApplication;
  protected
    procedure Execute; override;
  public
    FForm : TForm2;
    procedure OnNotify (Sender : TObject);
  end;

  TForm1 = class(TForm)
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
  private
    FFormThread : TFormThread;
    FTerminateByClose : Boolean;
    procedure OnTerminate (Sender : TObject);
  end;

{...}

implementation

{...}

{TForm1}

procedure TForm1.FormCreate(Sender: TObject);
begin
  FTerminateByClose := False;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  if not Assigned (FFormThread) then
  begin
    FFormThread := TFormThread.Create(True);
    FFormThread.FreeOnTerminate := True;
    FFormThread.OnTerminate := OnTerminate;
    FFormThread.Resume;
  end;
end;

procedure TForm1.OnTerminate(Sender: TObject);
begin
  if not FTerminateByClose then
    FFormThread := nil;
end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  If Assigned (FFormThread) then
  begin
    FTerminateByClose := True;
    SendMessage (FFormThread.FForm.Handle, WM_CLOSE, 0, 0);
    FFormThread.FreeInstance;
    FFormThread := nil;
  end;
  Action := caFree;
end;

{ TFormThread }

procedure TFormThread.Execute;
begin
  FApplication := TApplication.Create(nil);
  try
    FApplication.Initialize;
    FForm := TForm2.Create(FApplication);
    FForm.OnNotifyParent := OnNotify;
    FForm.Show;
    while (not Terminated) do
    begin
      Sleep (1);
      FApplication.ProcessMessages;
    end;
    FApplication.ShowHint := False;
    FApplication.Destroying;
    FApplication.DestroyComponents;
  finally
    FreeAndNil (FApplication);
  end;
end;

procedure TFormThread.OnNotify(Sender: TObject);
begin
  Terminate;
end;

interface

{...}

type
  TForm2 = class(TForm)
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
  private
    FOnNotify : TNotifyEvent;
  public
    property OnNotifyParent : TNotifyEvent read FOnNotify write FOnNotify;
  end;

{...}

implementation

{TForm2}

procedure TForm2.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action := caHide;
  if Assigned (FOnNotify) then
    FOnNotify(Self);
end;