Top: cross-sectional cut at z = 0 of vorticity squared (in logarithmic scale) for the thin-layer case at three different simulation times indicated above each panel. Bottom: same, but for the current density squared at z ≈ L/2. The straight lines seen in some panels are visualization artifacts at the boundaries of different AMR patches. These artifacts are not present in the actual simulation data. The full temporal evolution is available as an online movie.