code: public synchronized void clear() {
          List<Rectangle> flatBoxes = new ArrayList<>();
          final Collection<List<Rectangle>> boxesCollection = boxes.values();
          for (List<Rectangle> box : boxesCollection) {
              flatBoxes.addAll(box);
          }
          boxes.clear();
          if(parent!=null) JFXUtilities.runInFX(()-> getParentChildren(parent).removeAll(flatBoxes));
      }
docs: clear highlights