Function transcript_frames 

async fn transcript_frames(
    state: &TransportState,
    slot: &SessionSlot,
    cwd: &Path,
    from: u64,
    to: u64,
) -> Vec<SeatFrameWire>

The frames a range ask is answered with once the seat’s own window has shed them: the conversation’s own rows, numbered to END at the range’s end.

What a frame was numbered is minted in memory and written down nowhere. So a range that reaches past the window takes the transcript’s newest rows - the stretch the client is reading, read back off the file - and numbers them so the last one is the ask’s own end: the client resumes exactly where it was reading rather than asking again for a stretch that can never arrive, and the numbers it holds stay the numbers of one sequence rather than restarting at one.