title: SlidingEventTimeWindows does not produce any output
body: <p>I have a stream execution configured as</p>
  <pre class="lang-java prettyprint-override"><code>StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
  
  DataStreamSource&lt;Record&gt; stream = env.addSource(new FlinkKafkaConsumer(
      SystemsCpu.TOPIC,
      ConfluentRegistryAvroDeserializationSchema.forGeneric(SystemsCpu.SCHEMA, registry),
      config)
      .setStartFromLatest());
  
  DataStream&lt;Anomaly&gt; anomalies = stream
      .keyBy(x -&gt; x.get(&quot;host&quot;).toString())
      .window(SlidingEventTimeWindows.of(Time.minutes(20), Time.seconds(20))) // produces output with TumblingEventTimeWindows
      .process(new AnomalyDetector())
      .name(&quot;anomaly-detector&quot;);
  
  </code></pre>
  <pre class="lang-java prettyprint-override"><code>public class AnomalyDetector extends ProcessWindowFunction&lt;Record, Anomaly, String, TimeWindow&gt; {
  
    @Override
    public void process(String key, Context context, Iterable&lt;Record&gt; input, Collector&lt;Anomaly&gt; out) {
      var anomaly = new Anomaly();
      anomaly.setValue(1.0);
      out.collect(anomaly);
    }
  }
  </code></pre>
  <p>However for some reason <code>SlidingEventTimeWindows</code> does not produce any output to be processed by the <code>AnomalyDetector</code> (i.e. process is not triggered at all). If I use, for example, <code>TumblingEventTimeWindows</code> it works as expected.</p>
  <p>Any ideas what might be causing this? Am I using <code>SlidingEventTimeWindows</code> incorrectly?</p>