docs: public void add(Event<T> windowEvent) {
      // watermark events are not added to the queue.
      if (windowEvent.isWatermark()) {
        LOG.fine(String.format("Got watermark event with ts %d", windowEvent.getTimestamp()));
      } else {
        queue.add(windowEvent);
      }
      track(windowEvent);
      compactWindow();
    }