Rings of samples generated outside the patch's own edge.
A central difference needs a neighbor on both sides, so without any border the edge row falls back to a one-sided difference — half the gradient over half the span — and every patch boundary draws as a hairline of mis-shaded ground. That is one ring. The second is the morph: a patch about to hand over to its parent has to shade like its parent as well as sit where its parent sits, and the parent's normal is a difference across two of this patch's cells, so the outermost vertex reaches two samples out.
Taking those rows from the neighboring patch is the other way to get them and is strictly worse: it makes a patch's geometry depend on which of its neighbors happen to be loaded, which is an order dependency in a system whose whole premise is that a patch is a pure function of its address.
The cost is (69² − 65²) / 65² = 12.7% more samples, and it buys a patch that needs to know nothing about anybody.
universe/src/terrain.ts:998