sclass VGridFinderGUI {
  BufferedImage img;
  ImageSurface is;
  JSlider slider;
  int ww = 10;
  
  *(BufferedImage *img) {}

  void show {
    is = showImage(img);
    slider = jverticalLiveSlider(1, 50, ww, voidfunc(Int value) {
      ww = value;
      split();
    });
    addToWindowRight(is, withMargin(sliderWithValueLabel(slider)));
    split();
  }
  
  void split {
    new L<Rect> rects;
    int w = img.getWidth(), h = img.getHeight();
    for (int y = 0; y < h; y += ww)
      rects.add(new Rect(0, y, w, min(ww, h-y)));
    is.setImage(mergeImagePartsVertically(new RGBImage(img), rects));
  }
}

Began life as a copy of #1015499