Reading about the batch optimization, I suspect there'll be trouble ahead for implementing window functions, unless those details were dropped for simplicity.
How so? Window functions work exactly the same way. Postgres processes them one row at a time, and you can batch them the same way as you would with sum.
Window functions do make parallel queries more difficult, but that's a different story.
I guess regardless of if the calculations are batched or not, all the rows would have to be processed before the window function results could be determined.