diff --git a/tests/unit/terrain-grid-resize-controls.test.ts b/tests/unit/terrain-grid-resize-controls.test.ts
new file mode 100644
index 00000000..73fc7c34
--- /dev/null
+++ b/tests/unit/terrain-grid-resize-controls.test.ts
@@ -0,0 +1,68 @@
+import { describe, expect, it } from "vitest";
+
+import { createTerrain } from "../../src/document/terrains";
+import {
+  resizeTerrainGridFromBorderDrag,
+  resolveTerrainGridResizeSideFromLocalPosition
+} from "../../src/viewport-three/terrain-grid-resize-controls";
+
+describe("terrain grid resize viewport controls", () => {
+  it("resolves cardinal sides from local border hover positions", () => {
+    const terrain = createTerrain({
+      id: "terrain-grid-hover-sides",
+      sampleCountX: 5,
+      sampleCountZ: 5,
+      cellSize: 1
+    });
+
+    expect(resolveTerrainGridResizeSideFromLocalPosition(terrain, 0.1, 2, 0.25)).toBe(
+      "west"
+    );
+    expect(resolveTerrainGridResizeSideFromLocalPosition(terrain, 3.9, 2, 0.25)).toBe(
+      "east"
+    );
+    expect(resolveTerrainGridResizeSideFromLocalPosition(terrain, 2, 0.1, 0.25)).toBe(
+      "south"
+    );
+    expect(resolveTerrainGridResizeSideFromLocalPosition(terrain, 2, 3.9, 0.25)).toBe(
+      "north"
+    );
+    expect(resolveTerrainGridResizeSideFromLocalPosition(terrain, 2, 2, 0.25)).toBeNull();
+  });
+
+  it("drags east and north borders by rounded cell deltas", () => {
+    const terrain = createTerrain({
+      id: "terrain-grid-drag-east-north",
+      sampleCountX: 5,
+      sampleCountZ: 5,
+      cellSize: 1
+    });
+
+    const eastResized = resizeTerrainGridFromBorderDrag(terrain, "east", 1.6);
+    const northResized = resizeTerrainGridFromBorderDrag(terrain, "north", -1.2);
+
+    expect(eastResized.sampleCountX).toBe(7);
+    expect(eastResized.sampleCountZ).toBe(5);
+    expect(eastResized.position).toEqual(terrain.position);
+    expect(northResized.sampleCountX).toBe(5);
+    expect(northResized.sampleCountZ).toBe(4);
+    expect(northResized.position).toEqual(terrain.position);
+  });
+
+  it("drags west and south borders by moving the terrain origin", () => {
+    const terrain = createTerrain({
+      id: "terrain-grid-drag-west-south",
+      sampleCountX: 5,
+      sampleCountZ: 5,
+      cellSize: 1
+    });
+
+    const westResized = resizeTerrainGridFromBorderDrag(terrain, "west", 2);
+    const southResized = resizeTerrainGridFromBorderDrag(terrain, "south", -2);
+
+    expect(westResized.sampleCountX).toBe(7);
+    expect(westResized.position.x).toBe(terrain.position.x - 2);
+    expect(southResized.sampleCountZ).toBe(3);
+    expect(southResized.position.z).toBe(terrain.position.z + 2);
+  });
+});