static ImageSurface ImageSurfaceCheckerBoardBackground(ImageSurface is) {
  if (is.drawBackground != null) ret is;
  is.drawBackground = (w, h, g) -> {
    var cb = new CheckerBoard2().w(w).h(h);
    double time = sysSeconds();
    cb.shiftX(iround(time*8));
    cb.shiftY(iround(time*8));
    cb.drawOn(g);
  };
  setDoubleBuffered(is);
  repaintEvery(100, is);
  ret is;
}