Our zlib library was very roughly ported from CRuby many years ago, as described in #4835 (comment). Unfortunately this logic has not kept up with changes and improvements in CRuby, leading to issues like #6606.

We should try to align our logic with CRuby and get all available tests passing, to avoid future incompatibility reports.

The original folks that worked on this may be interested in helping: @nahi @ymnk.