Esme Povirk (@madewokherd) commented about dlls/windowscodecs/imgfactory.c:
if (cached_stream) {
LARGE_INTEGER move;
ULARGE_INTEGER pos;
/* Move wrapper stream to match original stream position. */
move.QuadPart = 0;
hr = IStream_Seek(cached_stream, move, STREAM_SEEK_CUR, &pos);
if (SUCCEEDED(hr))
hr = IStream_Seek(cached_stream, move, STREAM_SEEK_SET, NULL);
What's this seek for?