def next_offset(self) -> int:
        """
        The offset for the next frame to send.

        This is used to determine the space needed for the frame's `offset` field.
        """
        try:
            return self._pending[0].start
        except IndexError:
            return self._buffer_stop

        The offset for the next frame to send.

        This is used to determine the space needed for the frame's `offset` field.
        
src/aioquic/quic/stream.py