During implementation of the mod_gssapi test module I found that I wasn't getting an WWW-Authenticate header back so had no chance to do negotiation.
AFAICT this boiled down to the order of the modules: mod_auth_gssapi needs to be loaded before mod_auth_mellon.
Opened an issue with mod_auth_gssapi, https://github.com/modauthgssapi/mod_auth_gssapi/issues/32
Closing this.
resolution: => wontfix status: new => closed
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