Github user aljoscha commented on the issue:
https://github.com/apache/flink/pull/6224
I think the reasons at that time where:
- it changes the semantics of Triggers
- You can get into a situation where you call `Trigger.onEventTime()`, the
`Trigger` returns `FIRE` or `FIRE_AND_PURGE`, and then you don't actually have
any window contents to emit---
