I tested is and it is not "state cookie" but it is "absolute position" and it is "stateless".

> The I/O stack in Python 3 does not use C FILE streams, and this issue is not related to Windows. TextIOWrapper.tell returns a "cookie" based on the decoder state:

I will study: https://github.com/python/cpython/blob/3.8/Modules/_io/textio.c#L2589 - Thank you for reference.

See this test code (wrong seek is good - decoder has not state before - first line of program).
You can swap this two fragments and it still works.

print('seek 18446744073709554618')
with open('../s01_parser_eval/data/out-6976.txt') as pgn:
    pgn.seek(18446744073709554618)
    while pgn.tell() != 3003:
        pgn.readline()
        print(pgn.tell())

print()
print('seek 0')
with open('../s01_parser_eval/data/out-6976.txt') as pgn:
    pgn.seek(0)
    while pgn.tell() != 18446744073709554618:
        pgn.readline()
        print(pgn.tell())
    pgn.readline()
    print('next', pgn.tell())

print('seek 18446744073709554618')
with open('../s01_parser_eval/data/out-6976.txt') as pgn:
    pgn.seek(18446744073709554618)
    while pgn.tell() != 3003:
        pgn.readline()
        print(pgn.tell()))