fn poll_seek(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        pos: SeekFrom,
    ) -> Poll<io::Result<u64>> {
        ready!(self.as_mut().flush_buf(cx))?;
        self.project().inner.poll_seek(cx, pos)
    }
Seek to the offset, in bytes, in the underlying writer.

Seeking always writes out the internal buffer before seeking.
futures-util/src/io/buf_writer.rs