The scrollback middleware saves a services output, and sends it to any new clients that connect. It stores a maximum of 1,000 lines (denoted by a raw \n), of unlimited length. If the history is full new lines will push out old ones. The output is handled by lines to try and prevent split escape sequences.
At this time the full list of services which use this middleware is unknown, but it is likely that shell, and shellrun2 use it.
Info from the old developer docs.