“Mutter is a window and compositing manager that displays and
manages your desktop via OpenGL. Mutter combines a sophisticated
display engine using the Clutter toolkit with solid
window-management logic inherited from the Metacity window
manager.While Mutter can be used stand-alone, it is primarily intended
to be used as the display core of a larger system such as GNOME
Shell. For this reason, Mutter is very extensible via plugins,
which are used both to add fancy visual effects and to rework the
window management behaviors to meet the needs of the
environment.