This is a flat, top-down re-implementation of the same physics as the 3D version, but the model itself is simplified rather than just re-projected: a smaller lattice, a lighter per-frame walker budget tuned for a plain 2D canvas, and a single flat colour ramp instead of per-cell 3D relief. The underlying rule is still genuine diffusion-limited aggregation — vapour-molecule "walkers" random-walk over the pane and freeze onto the growing ice cluster only when adjacent to it and only with some sticking probability — seeded at defect points the way real frost nucleates at scratches and dust rather than on clean glass.
A live box-counting pass estimates the fractal dimension of the pattern you're growing; real 2D DLA clusters measure D ≈ 1.71, and you should see this simulation settle in the 1.5–1.7 range as the pattern fills in.