One of the forks of Metacity is known as Mutter, because it’s Metacity with Clutter support. It’s used by the forthcoming gnome-shell project.
In a recent email to d-d-l, Owen Taylor gave two goals for the 2.28 release:
That Mutter should be developed using the GNOME infrastructure; and
That users will be able to choose between gnome-shell and [...]
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