unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs;

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
  private
    { Private declarations }
    FThread: TThread;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

type
  TMyThread = class(TThread)
  protected
    procedure Execute; override;
  end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  FThread.Free;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FThread := TMyThread.Create;
end;

{ TMyThread }

procedure TMyThread.Execute;
begin
  Synchronize(
    procedure
    var
      i: Integer;
    begin
      i := GetTickCount;
      while (GetTickCount - i) < 3000 do
      begin
        Application.ProcessMessages
      end
    end
  );
end;

end.