TForm.Style:=bsNone

setWindowLong (handle,GWL_STYLE,getWindowLong(handle, GWL_STYLE) and not WS_CAPTION);

width:=width+1;

width:=width-1;

{ Private Declaration }
procedure CreateParams(var Params : TCreateParams); override;
...
procedure TForm1.CreateParams(var Params : TCreateParams);
begin
inherited Createparams(Params);
with Params do
Style := (Style or WS_POPUP) and not WS_DLGFRAME;
end;

procedure TForm1.HideTitlebar;
var
 Style: Longint;
begin
 if BorderStyle = bsNone then Exit;
 Style := GetWindowLong(Handle, GWL_STYLE);
 if (Style and WS_CAPTION) = WS_CAPTION then
 begin
  case BorderStyle of
  bsSingle,
  bsSizeable: SetWindowLong(Handle, GWL_STYLE, Style and
  (not (WS_CAPTION)) or WS_BORDER);
  bsDialog: SetWindowLong(Handle, GWL_STYLE, Style and
  (not (WS_CAPTION)) or DS_MODALFRAME or WS_DLGFRAME);
  end;
  Height := Height - GetSystemMetrics(SM_CYCAPTION);
  Refresh;
 end;
end;
procedure TForm1.ShowTitlebar;
var
 Style: Longint;
begin
 if BorderStyle = bsNone then Exit;
 Style := GetWindowLong(Handle, GWL_STYLE);
 if (Style and WS_CAPTION) <> WS_CAPTION then
 begin
  case BorderStyle of
  bsSingle,
  bsSizeable: SetWindowLong(Handle, GWL_STYLE, Style or WS_CAPTION or
  WS_BORDER);
  bsDialog: SetWindowLong(Handle, GWL_STYLE,
  Style or WS_CAPTION or DS_MODALFRAME or WS_DLGFRAME);
  end;
  Height := Height + GetSystemMetrics(SM_CYCAPTION);
  Refresh;
 end;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
 HideTitlebar;
end;
procedure TForm1.Button2Click(Sender: TObject);
begin
 ShowTitlebar;
end;