private
  { Private declarations }
  procedure WMNCPaint(var Msg : TWMNCPaint); message WM_NCPAINT;

procedure TForm1.WMNCPaint(var Msg: TWMNCPaint);
var
  dc: hDc;
  Pen: hPen;
  OldPen: hPen;
  OldBrush: hBrush;
begin
  inherited;
  dc := GetWindowDC(Handle);
  msg.Result := 1;
  Pen := CreatePen(PS_SOLID, 1, RGB(255, 0, 0));
  OldPen := SelectObject(dc, Pen);
  OldBrush := SelectObject(dc, GetStockObject(NULL_BRUSH));
  Rectangle(dc, 0,0, Form1.Width, Form1.Height);
  SelectObject(dc, OldBrush);
  SelectObject(dc, OldPen);
  DeleteObject(Pen);
  ReleaseDC(Handle, Canvas.Handle);
end;