static MouseAdapter onMouseDown(Component c, Runnable r) {
  ret onMouseDown(c, runnableToIVF1(r));
}

static MouseAdapter onMouseDown(Component c, IVF1<MouseEvent> f) {
  ret c == null || f == null ?: swing(-> {
    MouseAdapter ma = new {
      public void mousePressed(MouseEvent e) {
        if (e.getButton() == MouseEvent.BUTTON1)
          f.get(e);
      }
    };
    c.addMouseListener(ma);
    ret ma;
  });
}

Began life as a copy of #1015170