code: public void add(Event<T> windowEvent) {
          // watermark events are not added to the queue.
          if (!windowEvent.isWatermark()) {
              queue.add(windowEvent);
          } else {
              LOG.debug("Got watermark event with ts {}", windowEvent.getTimestamp());
          }
          track(windowEvent);
          compactWindow();
      }
docs: Tracks a window event
  
  @param windowEvent the window event to track