See here in the design document:

https://docs.google.com/document/d/1hyafrsPj0iuYFu2dm0ziGttkzE_GKbrQHRPsN81kYds/edit#heading=h.34gf9rx1vabj

Make the changes in this class:

https://github.com/10gen/mongo-enterprise-modules/blob/master/src/streams/exec/window_aware_group_operator.cpp

Example in unit tests: https://github.com/10gen/mongo-enterprise-modules/blob/master/src/streams/exec/tests/window_aware_operator_test.cpp#L493

Take a look at this proof of concept:

https://github.com/10gen/mongo-enterprise-modules/blob/streams/checkpointhackathon/src/streams/exec/group_operator.cpp#L127