The Mir 1.2 release was aiming to make it easier to develop Mir servers with custom Wayland extensions easier, but in dog feeding the work, Canonical's long-time Mir developer Alan Griffiths realized some shortcomings in the experience.

Fortunately, Alan quickly realized the issues and has already been able to commit these improvements to improve the experience for implementing new Wayland extensions for Mir servers.

Should you be interested in Mir for implementing additional Wayland extensions, you can learn more via this Discourse post.