Temporal evolution of the gas surface density Σ (shown in a logarithmic colour gradient) in a simulation with α_DZ = 10⁻³ and α_MRI = 10⁻¹. In each panel, the depicted portion of the disk is centred on TOI-216b. A rectangular projection of azimuthal coordinates is used to highlight disk features orbiting at the same radial separation. Streamlines of the gas flow are overlaid as white oriented curves. Individual panels are labelled with the migration time t_mig (corresponding to Fig. 1) and the instantaneous orbital radius of the planet r_p. Panels a–d capture important phases of migration of TOI-216b towards the viscosity transition, panel e shows a state shortly after the migration stalls and panel f shows the final state of our simulation. The figure is also available as an online movie showing the temporal evolution between t_mig = 16 000 and 17 000 P_orb.