Pick one and press Start, then click Next › to step through it.
Each band turning green in turn as the dashed watermark line sweeps past its right edge.
The base case: group events by event time into non-overlapping buckets, and let the watermark decide when each bucket is done.
Events at 1..14 land in [0,4), [4,8), [8,12), [12,16); the watermark trails the newest event by one, firing each window as it crosses the end.