From Gentoo Wiki
Testing version on stable system
This discussion is done.
Have removed this paragraph since it's no longer necessary.
- "This will install MATE 1.6 if you run a stable system and MATE 1.8 if you run a testing system; if you want MATE 1.8 on a stable system, use these /etc/portage/package.accept_keywords entries and then run the above command."
Builds on ~arm64 (Raspberry Pi3) after keywording the required packages and fixing USE=gnome-keyring being masked. Its not failed yet but so far, testing is fairly minimal.