Where to discuss options/scope of an issue (and its pull request)?

[Jeanette Winzenburg]

as an example take f.i.  
https://bugs.openjdk.java.net/browse/JDK-8129123 (and its pull request  
https://github.com/openjdk/jfx/pull/136 with a good starter to a  
solution) has open questions - for me, at least :)

Places to discuss: the pull request? the mailing list? the issue?  
Where would such a debate - assuming there's some lasting value in it  
- be best done such that it wouldn't be lost?

-- Jeanette