* other thread (last thread modifying the state protected by the monitor) takes over the
   * responsibility of signalling the next waiter.
   *
   * <p>This method must not be called from within a beginWaitingFor/endWaitingFor block, or else
   * the current thread's guard might be mistakenly signalled, leading to a lost signal.
   */
  @GuardedBy("lock")
  private void signalNextWaiter() {