static float darkenScreenArea_factor = 0.5f;

static JWindow darkenScreenArea(Rectangle r, double seconds) {
  ret disposeWindowAfter(iround(seconds*1000),
    darkenScreenArea(r));
}

static JWindow darkenScreenArea(final Rectangle r) {
  ret (JWindow) swingAndWait(func {
    Rectangle r2 = maxWindowBounds().intersection(r);
    if (neq(r, r2)) warn("darkenScreenArea: Rectangle cropped");
    if (r2.isEmpty()) null;
    
    BufferedImage img = shootScreen2(r2);
    RescaleOp op = new RescaleOp(darkenScreenArea_factor, 0, null);
    img = op.filter(img, null);
    
    JLabel label = new JLabel(imageIcon(img));
    new JWindow w;
    w.setBounds(r2);
    w.add(label);
    w.setAlwaysOnTop(true);
    w.setVisible(true);
    ret w;
  });
}

Began life as a copy of #1005626