In order for me to push the fixed bugs along I had to rename the project to zlib.managed, also in fixing the bugs I created a helper class that compresses the data simpler and does not require users to need to use ZInputStream or ZOutputStream themselves called MemoryZlib which has members Compress and Decompress that allows them to set:

1. Compression levels,
2. Pass in the output as the second parameter to the method.
3. The first parameter is the actual raw data.

Also if you want to archive this and let the people who fork zlib.net or even somehow get the normal zlib.net to update to say Use the package 'zlib.managed' instead of this one. That package major bug fixes and has helper methods to simplify all of your zlib usage.
https://github.com/Elskom/zlib.managed/

Also I see you took the original zlib.net like what I did, and simply just made other changes to it.

Also I found some similarities as well, except what I did on mine was place the Compress and Decompress methods in an MemoryZlib class.

Also an IsCompressedByZlib that would be nice to add on mine too, if you would like you can always replace this package with mine, it contains a lot of bug fixes and can also help further develop zlib.net if 1 version is developed to instead of a ton of zlib.net based ones that contains fixes, and lacks other major ones as well at the same time.

Also my version contains some code cleanup, the csproj's are the new sdk style ones and because of that requires little to no issues updating them.