[Maria-discuss] MariaDB's tracking of MySQL changes

Can someone explain the relationship between MySQL and MariaDB code updates? I've read that MariaDB is a drop-in *replacement* for MySQL. But I've also read that MariaDB treats MySQL as an upstream code base. So what's the deal? Does MariaDB have alternative implementations for just some of MySQL's components, and when a new MySQL release comes out, MariaDB updates just those components for which it doesn't have its own versions? [This is a re-post from the #maria IRC channel, where I got no answer.] Thanks, Christian