Created attachment 441452 [details] build.log I had MATE-1.12 running with gtk2 and did a rebuild with gtk3. Everything built fine with the exeption of mate-extra/mate-indicator-applet-1.12.1. Seems it's still looking vor dev-libs/libindicator-12.10.1-r201 though -r301 is installed?
Created attachment 441454 [details] emerge --info
Fixed in 1.12.1-r1 in efbcbb94d7d37fc2a02ee3aba88ff7648d5ffbae. Thanks for the report/testing.