Вы можете попробовать следующее:

procedure TForm1.HideTitlebar;
var
  Save : Longint;
begin
  if BorderStyle=bsNone then Exit;
  Save:=GetWindowLong(Handle, GWL_STYLE);
  if (Save and WS_CAPTION)=WS_CAPTION then begin
    case BorderStyle of
      bsSingle,
      bsSizeable : SetWindowLong(Handle, GWL_STYLE, Save and
                   (Not WS_CAPTION) or WS_BORDER);
      bsDialog : SetWindowLong(Handle, GWL_STYLE, Save and
                 (Not WS_CAPTION) or DS_MODALFRAME or WS_DLGFRAME);
    end;
    Height:=Height-GetSystemMetrics(SM_CYCAPTION);
    Refresh;
  End;
end;

procedure TForm1.ShowTitlebar;
var
  Save : Longint;
begin
  if BorderStyle=bsNone then Exit;
  Save:=GetWindowLong(Handle, GWL_STYLE);
  if (Save and WS_CAPTION)<>WS_CAPTION then begin
    case BorderStyle of
      bsSingle,
      bsSizeable : SetWindowLong(Handle, GWL_STYLE, Save or WS_CAPTION or
                   WS_BORDER);
      bsDialog : SetWindowLong(Handle, GWL_STYLE, Save or WS_CAPTION or
        DS_MODALFRAME or WS_DLGFRAME);
    end;
    Height:=Height+getSystemMetrics(SM_CYCAPTION);
    Refresh;
  end;
end;