More info: https://en.opensuse.org/SDB:Vendor_change_update Basically to emulate the behaviour with current features one would have to mask all packages in all repos and only unmask the packages which he wants to merge while specifying the exact repo. As no one wants to go that way, the only sensible way currently is to pay a close attention when doing any emerge and watch for repo change in the emerge summary. Having multiple overlays installed is a quite natural thing in Gentoo, but using them is really inconvenient and error-prone currently.
