Mutter is a Wayland display server and X11 window manager and compositor
library.
It implements the compositor side of the Wayland core protocol as well
as various protocol extensions. It also has functionality related to
running X11 applications using Xwayland. When used on top of Xorg it acts
as a X11 window manager and compositing manager. It contains functionality
related to, among other things, window management, window compositing,
focus tracking, workspace management, keybindings and monitor configuration.