static int updateCanvas_retryInterval = 50;

// if makeImg returns null, it is recalled after a delay
static void updateCanvas(final Canvas canvas, fO makeImg) {
  awtIfNecessary {
    if (canvas.updating || canvas.getWidth() == 0) ret;
    canvas.updating = true;
    try {
      BufferedImage img = asBufferedImage(callF(makeImg, canvas.getWidth(), canvas.getHeight()));
      if (img != null) {
        canvas.setImage(img);
        canvas.updating = false;
      } else
        awtLater(updateCanvas_retryInterval, r {
          canvas.updating = false;
          updateCanvas(canvas, makeImg);
        });
    } catch e {
      canvas.updating = false;
      throw rethrow(e);
    }
  }
}

static void updateCanvas(final Canvas canvas) {
  if (canvas != null) canvas.update();
}

Began life as a copy of #1007284