Happens to address part of https://github.com/matrix-org/matrix-public-archive/issues/271
but made primarily as a follow-up to https://github.com/matrix-org/matrix-public-archive/pull/239
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Only 42% rooms on the `matrix.org` room directory are `world_readable` which means we will get pages of rooms that are half-empty most of the time if we just naively fetch 9 rooms at a time.
Ideally, we would be able to just add a filter directly to `/publicRooms` in order to only grab the `world_readable` rooms and still get full pages but the filter option doesn't allow us to slice by `world_readable` history visibility.
Instead, we have to paginate until we get a full grid of 9 rooms, then make a final `/publicRooms` request to backtrack to the exact continuation point so next page won't skip any rooms in between.
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We had empty spaces in the grid before because some rooms in the room directory are private which we filtered out before. But that was a much more rare experience since only 2% of rooms were private .
Eric Eastwood