static BoolVar componentShowingVar(JComponent component) {
  ret swing(-> {
    BoolVar flag = new(component.isShowing());
    
    component.addAncestorListener(new AncestorListener {
      public void ancestorAdded(AncestorEvent event) {
        if (flag!) print("Warning: bindToComponent logic failure");
        flag.set(true);
      }
  
      public void ancestorRemoved(AncestorEvent event) {
        if (!flag!) print("Warning: bindToComponent logic failure");
        flag.set(false);
      }
  
      public void ancestorMoved(AncestorEvent event) {
      }
    });
    
    ret flag;
  });
}

Began life as a copy of #1003557