SlidingWindow.scala

      case class SomeEvent(value: Long)

      val events = Source
        .tick(0 seconds, 250 millis, "")
        .zipWithIndex
        .map { case (_, l) =>
            SomeEvent(l)
        }

      val group = Flow[SomeEvent].groupedWithin(100, 500 millis) // +/- 2 events per group
      val slide = Flow[Seq[SomeEvent]].sliding(3, 1) // 3 groups per slide, step size = 1
      val exe = events
        .via(group)
        .via(slide)
        .map(_.flatten)
        .runWith(Sink.foreach(println))

      try Await.result(exe, 6 seconds)
      catch { case _: Throwable => }

This will generate something like this:

Vector(SomeEvent(0), SomeEvent(1), SomeEvent(2), SomeEvent(3), SomeEvent(4), SomeEvent(5), SomeEvent(6))
Vector(SomeEvent(2), SomeEvent(3), SomeEvent(4), SomeEvent(5), SomeEvent(6), SomeEvent(7))
Vector(SomeEvent(5), SomeEvent(6), SomeEvent(7), SomeEvent(8), SomeEvent(9), SomeEvent(10))
Vector(SomeEvent(7), SomeEvent(8), SomeEvent(9), SomeEvent(10), SomeEvent(11))
Vector(SomeEvent(8), SomeEvent(9), SomeEvent(10), SomeEvent(11), SomeEvent(12), SomeEvent(13))
Vector(SomeEvent(11), SomeEvent(12), SomeEvent(13), SomeEvent(14), SomeEvent(15), SomeEvent(16))
Vector(SomeEvent(12), SomeEvent(13), SomeEvent(14), SomeEvent(15), SomeEvent(16), SomeEvent(17), SomeEvent(18))
Vector(SomeEvent(14), SomeEvent(15), SomeEvent(16), SomeEvent(17), SomeEvent(18), SomeEvent(19))
Vector(SomeEvent(17), SomeEvent(18), SomeEvent(19), SomeEvent(20), SomeEvent(21))

If you leave the .map(_.flatten) out, then you can still see the individual groups. For example:

Vector(Vector(SomeEvent(0), SomeEvent(1)), Vector(SomeEvent(2), SomeEvent(3), SomeEvent(4)), Vector(SomeEvent(5), SomeEvent(6)))
Vector(Vector(SomeEvent(2), SomeEvent(3), SomeEvent(4)), Vector(SomeEvent(5), SomeEvent(6)), Vector(SomeEvent(7)))
Vector(Vector(SomeEvent(5), SomeEvent(6)), Vector(SomeEvent(7)), Vector(SomeEvent(8), SomeEvent(9), SomeEvent(10)))
Vector(Vector(SomeEvent(7)), Vector(SomeEvent(8), SomeEvent(9), SomeEvent(10)), Vector(SomeEvent(11)))
Vector(Vector(SomeEvent(8), SomeEvent(9), SomeEvent(10)), Vector(SomeEvent(11)), Vector(SomeEvent(12), SomeEvent(13)))
Vector(Vector(SomeEvent(11)), Vector(SomeEvent(12), SomeEvent(13)), Vector(SomeEvent(14), SomeEvent(15)))
Vector(Vector(SomeEvent(12), SomeEvent(13)), Vector(SomeEvent(14), SomeEvent(15)), Vector(SomeEvent(16), SomeEvent(17), SomeEvent(18)))
Vector(Vector(SomeEvent(14), SomeEvent(15)), Vector(SomeEvent(16), SomeEvent(17), SomeEvent(18)), Vector(SomeEvent(19), SomeEvent(20)))
Vector(Vector(SomeEvent(16), SomeEvent(17), SomeEvent(18)), Vector(SomeEvent(19), SomeEvent(20)), Vector(SomeEvent(21)))