test: shim navigator in platformPaths tests for Node CI

Author: Dimillian

memory/mistakes.md

@@ -121,3 +121,13 @@ Rule: When converting effects between `useEffect` and `useLayoutEffect`, preserv
 Root cause: `autoScrollRef.current = true` still ran in `useEffect` after the new layout scroll pass, so first render on thread switch could evaluate stale `false`.
 Fix applied: Updated `src/features/messages/components/Messages.tsx` hook ordering/dependencies and added `re-pins to bottom on thread switch even when previous thread was scrolled up` in `src/features/messages/components/Messages.test.tsx`.
 Prevention rule: For scroll/anchor refs, pair layout-timing ref resets with layout-timing consumers and add regression coverage for cross-thread transitions.
+
+## 2026-02-07 21:14
+Context: CI `test-js` failure (`platformPaths.test.ts`)
+Type: mistake
+Event: New mobile platform tests mutated `navigator` directly without ensuring a `navigator` object exists in Node test environments.
+Action: Updated `withNavigatorValues` in `src/utils/platformPaths.test.ts` to create a temporary `globalThis.navigator` shim when missing, restore descriptors after each test, and clean up with `Reflect.deleteProperty`.
+Rule: Node-targeted unit tests must not assume browser globals exist; create and tear down explicit shims in helper setup.
+Root cause: The tests were authored assuming `navigator` is always available, but Vitest runs with `environment: node` in CI.
+Fix applied: Added a global-scope navigator shim path and descriptor-safe restore logic in `src/utils/platformPaths.test.ts`.
+Prevention rule: For tests that patch `navigator`, `window`, or `document`, guard setup with `typeof ... === \"undefined\"` and perform full teardown in `finally`.

src/utils/platformPaths.test.ts

@@ -1,28 +1,47 @@
 import { describe, expect, it } from "vitest";
 import { isMobilePlatform } from "./platformPaths";
 
+const globalScope = globalThis as typeof globalThis & { navigator?: Navigator };
+
 function withNavigatorValues(
   values: Partial<Pick<Navigator, "platform" | "userAgent">>,
   run: () => void,
 ) {
-  const originalPlatform = Object.getOwnPropertyDescriptor(navigator, "platform");
-  const originalUserAgent = Object.getOwnPropertyDescriptor(navigator, "userAgent");
-  Object.defineProperty(navigator, "platform", {
+  const hadNavigator = typeof globalScope.navigator !== "undefined";
+  if (!hadNavigator) {
+    Object.defineProperty(globalScope, "navigator", {
+      configurable: true,
+      writable: true,
+      value: {},
+    });
+  }
+
+  const activeNavigator = globalScope.navigator as Navigator;
+  const originalPlatform = Object.getOwnPropertyDescriptor(activeNavigator, "platform");
+  const originalUserAgent = Object.getOwnPropertyDescriptor(activeNavigator, "userAgent");
+  Object.defineProperty(activeNavigator, "platform", {
     configurable: true,
-    value: values.platform ?? navigator.platform,
+    value: values.platform ?? activeNavigator.platform ?? "",
   });
-  Object.defineProperty(navigator, "userAgent", {
+  Object.defineProperty(activeNavigator, "userAgent", {
     configurable: true,
-    value: values.userAgent ?? navigator.userAgent,
+    value: values.userAgent ?? activeNavigator.userAgent ?? "",
   });
   try {
     run();
   } finally {
     if (originalPlatform) {
-      Object.defineProperty(navigator, "platform", originalPlatform);
+      Object.defineProperty(activeNavigator, "platform", originalPlatform);
+    } else {
+      delete (activeNavigator as { platform?: string }).platform;
     }
     if (originalUserAgent) {
-      Object.defineProperty(navigator, "userAgent", originalUserAgent);
+      Object.defineProperty(activeNavigator, "userAgent", originalUserAgent);
+    } else {
+      delete (activeNavigator as { userAgent?: string }).userAgent;
+    }
+    if (!hadNavigator) {
+      Reflect.deleteProperty(globalScope, "navigator");
     }
   }
 }