// f: (int w, int h) -> BufferedImage
static ImageSurface jcanvas(O f) {
  asertAWTThread();
  final new ImageSurface is;

  onResize(is, r {
    awtLater(is, delay, r {
      BufferedImage img = is.getImage();#
      int w = is.getWidth(), h = is.getHeight();
      if (img.getWidth() != w || img.getHeight() != h)
        is.setImage(toBufferedImage(callF(f, w, h)));
    });
  });
}