PyEncoder.encode_to_file() writes buf[status:], where status is the number of encoded bytes. An encoder returning a complete buffer consequently writes nothing #10096

import tempfile
from PIL import ImageFile

class Encoder(ImageFile.PyEncoder):
    def encode(self, bufsize):
        return 3, 1, b"ABC"

with tempfile.TemporaryFile() as fp:
    Encoder("RGB").encode_to_file(fp.fileno(), 65536)
    fp.seek(0)
    print(fp.read())  # b''; expected b'ABC'

The same encoder through ImageFile._save() writes b'ABC' to BytesIO, but an empty buffer to a real file.

Write the encoded prefix at ImageFile.py:935, rather than slicing it off. This affects non-push Python encoders; the bundled QOI/BLP encoders use a different path.

Reproduced on Pillow 13.0.0.dev0, Python 3.13.15, with the original Python sources and C extension built from this checkout.