Mirrors are overlays that show a view-only PipeWire cast of any kind; screen, region, window.
The view-only part means that mouse input will not work on these. This is due to a PipeWire protocol limitation.
Mirrors can be initialized at runtime via the keyboard taskbar’s hamburger menu.
Mirrors are only supported on Wayland.
Note that not all Wayland compositors implement region / window casting.