Mass-weighted distribution of disc stars as a function of their initial and final semi-major axes at t = 1.5 Gyr. Top panels: results for the II-Dry-Orb1 (left) and II-Dry-Orb2 (right) simulations. Bottom panels: Mass-weighted average over all wet simulations (left) and a representative wet case on a direct orbit (right). The solid black line marks the 1:1 relation (i.e. no change in semi-major axis). We note that this is not a particle-based plot, but a binned, mass-weighted representation. The colour values (in M_⊙ pc⁻²) indicate the stellar mass redistributed from a given initial to a final semi-major axis bin. This binning method can yield higher values at intermediate radii than in the central region, due to differing amounts of radial migration and local mass distribution.