I noticed that if I use TimestampCombiner/EARLIEST for session windows that the watermark appears to get held up for sessions that never "close" (or that extend for a long time).
But if I use default (TimestampCombiner/END_OF_WINDOW) the watermark doesn't get held. Does this mean that the watermark is adjusted whenever windows are merged, even before they "close"? If that is the case, and I write a custom WindowFn, is this implication of this that I should never move the `end`/`maxTimestamp` of the new/merged window *backwards* in time?
