static S lineAtMouse() {
  ret swing(func -> S {
    Component c = (Component) componentAtMouse_gen();
    if (c instanceof JLabel)
      ret getText((JLabel) c);
      
    if (c instanceof JTextComponent) {
      JTextComponent tc = cast c;
      Point d = mouseToComponent(c);
      int idx = tc.viewToModel(d);
      S text = tc.getText();
      ret lineAroundIndex(text, idx);
    }
    ret "";
  });
}