Flushing the multithreaded implementation is different from the single-threaded implementation.

[rhpvorderman]

See pycompression/xopen#171 (comment)

[rhpvorderman]

The normal implementation uses Z_SYNC_FLUSH.

From the zlib manual

If the parameter flush is set to Z_SYNC_FLUSH, all pending output is flushed to the output buffer and the output is aligned on a byte boundary, so that the decompressor can get all input data available so far. (In particular avail_in is zero after the call if enough output space has been provided before the call.) Flushing may degrade compression for some compression algorithms and so it should be used only when necessary. This completes the current deflate block and follows it with an empty stored block that is three bits plus filler bits to the next byte, followed by four bytes (00 00 ff ff).

So this data can be decompressed using deflate raw blocks. Which is useful to know for testing the correct flush behaviour. Sync flush ends the deflate stream and may influence the compressed size because of that.