Evolution of the cross-section of the gas density (solid lines) and the gas tracer density (symbols and shaded regions) for the Kelvin–Helmoltz instability at different times (from blue to red from the start to the end of the simulation at t = 0.3 s). The profiles have been shifted vertically (each by 0.6 g cm⁻³) so that one may easily distinguish them from one another. The shaded regions are ±5σ, where σ has been estimated using a Poisson sampling noise. The gas tracers are accurately following the diffusion of the gas.