mutter
also: GNOME Shell compositor, GNOME Mutter
A display server and window manager for the GNOME desktop environment that manages windows, compositing, and rendering on modern Linux systems using Wayland or X11.
Mutter is the compositing window manager that powers GNOME. It handles drawing windows to the screen, managing window decorations, responding to keyboard and mouse input, and applying visual effects like shadows and animations.
Mutter supports both the legacy X11 and modern Wayland display protocols. When running under Wayland, it acts as the display server itself; under X11, it runs as a window manager on top of an existing X server.
Example: When you move a window in GNOME, resize it, or switch between workspaces, mutter is the software managing those operations and redrawing the screen. Most GNOME users never interact with mutter directly—it works invisibly in the background.