diff --git a/public/views/grid-setup.mjs b/public/views/grid-setup.mjs
index c120fe1..9c39817 100644
--- a/public/views/grid-setup.mjs
+++ b/public/views/grid-setup.mjs
@@ -183,8 +183,18 @@ export class Grid_Setup {
 				this.#corners[i] = { x: this.#corners[i].x + dx, y: this.#corners[i].y + dy };
 			} else {
 				const [a, b] = Grid_Setup.#MIDPOINT_EDGES[this.#drag_idx - 4];
-				this.#corners[a] = { x: this.#corners[a].x + dx, y: this.#corners[a].y + dy };
-				this.#corners[b] = { x: this.#corners[b].x + dx, y: this.#corners[b].y + dy };
+				// Project delta onto the edge's outward normal so the edge can
+				// only be pushed/pulled perpendicular to itself — never sheared.
+				const ex = this.#corners[b].x - this.#corners[a].x;
+				const ey = this.#corners[b].y - this.#corners[a].y;
+				const len = Math.sqrt(ex*ex + ey*ey);
+				if (len > 0) {
+					const nx = -ey / len;
+					const ny =  ex / len;
+					const proj = dx * nx + dy * ny;
+					this.#corners[a] = { x: this.#corners[a].x + nx * proj, y: this.#corners[a].y + ny * proj };
+					this.#corners[b] = { x: this.#corners[b].x + nx * proj, y: this.#corners[b].y + ny * proj };
+				}
 			}
 			this.#drag_prev_img = img_pos;
 			this.#draw();