fix(scan): prevent camera support render loop

src/app/scan/page.test.tsx

@@ -0,0 +1,61 @@
+import { StrictMode } from "react";
+import { renderToString } from "react-dom/server";
+import { fireEvent, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import ScanPage from "./page";
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+describe("ScanPage", () => {
+  it("renders camera controls without a render loop and remains usable after typing", () => {
+    vi.stubGlobal("isSecureContext", true);
+    vi.stubGlobal("navigator", {
+      mediaDevices: { getUserMedia: vi.fn() },
+    });
+
+    render(<StrictMode><ScanPage /></StrictMode>);
+
+    expect(screen.getByRole("button", { name: "Start camera" })).toBeEnabled();
+    expect(screen.getByRole("button", { name: "Go" })).toBeDisabled();
+
+    fireEvent.change(screen.getByLabelText("Or type the delivery note number"), {
+      target: { value: "dn26100401" },
+    });
+
+    expect(screen.getByLabelText("Or type the delivery note number")).toHaveValue("DN26100401");
+    expect(screen.getByRole("button", { name: "Go" })).toBeEnabled();
+    expect(screen.getByRole("button", { name: "Start camera" })).toBeEnabled();
+  });
+
+  it("keeps manual entry available on an insecure address without a camera", () => {
+    vi.stubGlobal("isSecureContext", false);
+    vi.stubGlobal("navigator", {});
+
+    render(<ScanPage />);
+
+    expect(screen.getByTestId("scan-insecure-warning")).toBeInTheDocument();
+    expect(screen.getByRole("button", { name: "Start camera" })).toBeDisabled();
+    fireEvent.change(screen.getByLabelText("Or type the delivery note number"), {
+      target: { value: "DN26100401" },
+    });
+    expect(screen.getByRole("button", { name: "Go" })).toBeEnabled();
+  });
+
+  it("hydrates the server page and enables the browser's camera controls", () => {
+    vi.stubGlobal("isSecureContext", true);
+    vi.stubGlobal("navigator", {
+      mediaDevices: { getUserMedia: vi.fn() },
+    });
+    const container = document.createElement("div");
+    container.innerHTML = renderToString(<ScanPage />);
+    document.body.appendChild(container);
+    expect(container.querySelector("button")).toBeDisabled();
+
+    render(<ScanPage />, { container, hydrate: true });
+
+    expect(screen.getByRole("button", { name: "Start camera" })).toBeEnabled();
+    expect(screen.getByLabelText("Or type the delivery note number")).toBeInTheDocument();
+  });
+});

src/app/scan/page.tsx

@@ -84,6 +84,25 @@ const STATUS_COPY: Record<string, string> = {
   cancelled: "Cancelled",
 };
 
+let supportSnapshot: ReturnType<typeof detectScannerSupport> | null = null;
+
+function getSupportSnapshot() {
+  const next = detectScannerSupport();
+  if (
+    !supportSnapshot ||
+    next.kind !== supportSnapshot.kind ||
+    next.camera !== supportSnapshot.camera ||
+    next.secureContext !== supportSnapshot.secureContext
+  ) {
+    supportSnapshot = next;
+  }
+  return supportSnapshot;
+}
+
+// Capabilities have no change event; React rechecks them on render.
+const subscribeToSupport = () => () => {};
+const getServerSupportSnapshot = () => null;
+
 export default function ScanPage() {
   const videoRef = React.useRef<HTMLVideoElement | null>(null);
   const canvasRef = React.useRef<HTMLCanvasElement | null>(null);
@@ -96,17 +115,15 @@ export default function ScanPage() {
   const wakeRef = React.useRef<WakeLockSentinelLike | null>(null);
 
   /**
-   * Capability snapshot. `useSyncExternalStore` with a null server
-   * snapshot rather than `useState` + `useEffect`: the value only
-   * exists in a browser (it reads `navigator` and `window`), so the
-   * first client render has to MATCH the server's null and then
-   * re-render with the real answer. Setting state inside an effect to
-   * get there is the React 19 `set-state-in-effect` anti-pattern.
+   * Match the server's null snapshot during hydration, then read the
+   * browser capabilities. Unchanged capabilities must return the same
+   * object: a fresh object on every read makes useSyncExternalStore
+   * keep re-rendering until React throws "Maximum update depth exceeded".
    */
   const support = React.useSyncExternalStore(
-    () => () => {},
-    () => detectScannerSupport(),
-    () => null,
+    subscribeToSupport,
+    getSupportSnapshot,
+    getServerSupportSnapshot,
   );
   const [cameraOn, setCameraOn] = React.useState(false);
   const [manual, setManual] = React.useState("");