A test of an arrangement needs more than one output. A nested niri instance gives one output with the name "winit", and that backend ignores a transform. The niri binary holds a headless backend, but the command line gives no access to it. This crate therefore holds a compositor of its own. It answers the same protocol on a socket of its own, and it applies the same rules: it keeps the scale factor between the limits, and it divides the physical size by the scale factor and removes the fraction. A test can then use many monitors, many scale factors and many transforms. The test gives the same result every time, and it needs no compositor and no hardware. The round trip test applies a plan and reads the outputs again. A second comparison with the same layout gives no more changes, which shows that the program reaches the state that the user asked for. The program also uses this compositor for a mode that shows the interface with no niri and no second monitor.