Sync hidden variant unit fields

This will be necessary for managing the 'display as' state.
..or is it?

spec/javascripts/stimulus/product_controller_test.js

@@ -0,0 +1,56 @@
+/**
+ * @jest-environment jsdom
+ */
+
+import { Application } from "stimulus";
+import product_controller from "../../../app/webpacker/controllers/product_controller";
+
+describe("ProductController", () => {
+  beforeAll(() => {
+    const application = Application.start();
+    application.register("product", product_controller);
+  });
+
+  describe("variant_unit_with_scale", () => {
+    beforeEach(() => {
+      document.body.innerHTML = `
+        <div data-controller="product">
+          <input id="variant_unit" name="[products][0][variant_unit]" value="weight">
+          <input id="variant_unit_scale" name="[products][0][variant_unit_scale]" value="1.0">
+          <select id="variant_unit_with_scale" name="[products][0][variant_unit_with_scale]">
+            <option selected="selected" value="weight_1">Weight (g)</option>
+            <option value="weight_1000">Weight (kg)</option>
+            <option value="volume_4.54609">Volume (gal)</option>
+            <option value="items">Items</option>
+          </select>
+        </div>
+      `;
+    });
+
+    describe("change", () => {
+      it("weight_1000", () => {
+        variant_unit_with_scale.selectedIndex = 1;
+        variant_unit_with_scale.dispatchEvent(new Event("change"));
+
+        expect(variant_unit.value).toBe("weight");
+        expect(variant_unit_scale.value).toBe("1000");
+      });
+
+      it("volume_4.54609", () => {
+        variant_unit_with_scale.selectedIndex = 2;
+        variant_unit_with_scale.dispatchEvent(new Event("change"));
+
+        expect(variant_unit.value).toBe("volume");
+        expect(variant_unit_scale.value).toBe("4.54609");
+      });
+
+      it("items", () => {
+        variant_unit_with_scale.selectedIndex = 3;
+        variant_unit_with_scale.dispatchEvent(new Event("change"));
+
+        expect(variant_unit.value).toBe("items");
+        expect(variant_unit_scale.value).toBe("");
+      });
+    })
+  });
+});