Github user wuchong commented on the issue:

    https://github.com/apache/flink/pull/4183
  
    Hi @fhueske , I like the `firstResultTimeOffset`, a good design !  👍 
    
    Regarding to the watermark and timestamp, it makes sense to me. And a 
simple approach come to my mind : assign a new 
`AssignerWithPunctuatedWatermarks`  after window aggregate. This will create an 
operator which ignores the upstream watermarks and assign new watermarks depend 
on the watermark function.
    
    The new watermark function can simply take the element's timestamp as the 
watermark.
    
    ```scala
    class TimestampAndWatermark[T] extends AssignerWithPunctuatedWatermarks[T] {
       
        override def checkAndGetNextWatermark(lastElement: T, 
extractedTimestamp: Long): Watermark = {
          new Watermark(extractedTimestamp)
        }
        
        override def extractTimestamp(element: T, originalTimestamp: Long): 
Long = originalTimestamp
      }
    ```
    



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