@Override
      public void hierarchyChanged(HierarchyEvent e) {
        if ((e.getChangeFlags() & HierarchyEvent.SHOWING_CHANGED) != 0) {
          if (myTargetEditor.isShowing()) {
            UndoUtil.enableUndoFor(myTargetEditor.getDocument());
          }
          else {
            UndoUtil.disableUndoFor(myTargetEditor.getDocument());
          }
        }
      }
@Override
      public void hierarchyChanged(HierarchyEvent e)