docs(frontend): React Router testing guide (#12145)

frontend/__tests__/router.md

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+# Testing with React Router
+
+## Overview
+
+React Router components and hooks require a routing context to function. In tests, we need to provide this context while maintaining control over the routing state.
+
+This guide covers the two main approaches used in the OpenHands frontend:
+
+1. **`createRoutesStub`** - Creates a complete route structure for testing components with their actual route configuration, loaders, and nested routes.
+2. **`MemoryRouter`** - Provides a minimal routing context for components that just need router hooks to work.
+
+Choose your approach based on what your component actually needs from the router.
+
+## When to Use Each Approach
+
+### `createRoutesStub` (Recommended)
+
+Use `createRoutesStub` when your component:
+- Relies on route parameters (`useParams`)
+- Uses loader data (`useLoaderData`) or `clientLoader`
+- Has nested routes or uses `<Outlet />`
+- Needs to test navigation between routes
+
+> [!NOTE]
+> `createRoutesStub` is intended for unit testing **reusable components** that depend on router context. For testing full route/page components, consider E2E tests (Playwright, Cypress) instead.
+
+```typescript
+import { createRoutesStub } from "react-router";
+import { render } from "@testing-library/react";
+
+const RouterStub = createRoutesStub([
+  {
+    Component: MyRouteComponent,
+    path: "/conversations/:conversationId",
+  },
+]);
+
+render(<RouterStub initialEntries={["/conversations/123"]} />);
+```
+
+**With nested routes and loaders:**
+
+```typescript
+const RouterStub = createRoutesStub([
+  {
+    Component: SettingsScreen,
+    clientLoader,
+    path: "/settings",
+    children: [
+      {
+        Component: () => <div data-testid="llm-settings" />,
+        path: "/settings",
+      },
+      {
+        Component: () => <div data-testid="git-settings" />,
+        path: "/settings/integrations",
+      },
+    ],
+  },
+]);
+
+render(<RouterStub initialEntries={["/settings/integrations"]} />);
+```
+
+> [!TIP]
+> When using `clientLoader` from a Route module, you may encounter type mismatches. Use `@ts-expect-error` as a workaround:
+
+```typescript
+import { clientLoader } from "@/routes/settings";
+
+const RouterStub = createRoutesStub([
+  {
+    path: "/settings",
+    Component: SettingsScreen,
+    // @ts-expect-error: loader types won't align between test and app code
+    loader: clientLoader,
+  },
+]);
+```
+
+### `MemoryRouter`
+
+Use `MemoryRouter` when your component:
+- Only needs basic routing context to render
+- Uses `<Link>` components but you don't need to test navigation
+- Doesn't depend on specific route parameters or loaders
+
+```typescript
+import { MemoryRouter } from "react-router";
+import { render } from "@testing-library/react";
+
+render(
+  <MemoryRouter>
+    <MyComponent />
+  </MemoryRouter>
+);
+```
+
+**With initial route:**
+
+```typescript
+render(
+  <MemoryRouter initialEntries={["/some/path"]}>
+    <MyComponent />
+  </MemoryRouter>
+);
+```
+
+## Anti-patterns to Avoid
+
+### Using `BrowserRouter` in tests
+
+`BrowserRouter` interacts with the actual browser history API, which can cause issues in test environments:
+
+```typescript
+// ❌ Avoid
+render(
+  <BrowserRouter>
+    <MyComponent />
+  </BrowserRouter>
+);
+
+// ✅ Use MemoryRouter instead
+render(
+  <MemoryRouter>
+    <MyComponent />
+  </MemoryRouter>
+);
+```
+
+### Mocking router hooks when `createRoutesStub` would work
+
+Mocking hooks like `useParams` directly can be brittle and doesn't test the actual routing behavior:
+
+```typescript
+// ❌ Avoid when possible
+vi.mock("react-router", async () => {
+  const actual = await vi.importActual("react-router");
+  return {
+    ...actual,
+    useParams: () => ({ conversationId: "123" }),
+  };
+});
+
+// ✅ Prefer createRoutesStub - tests real routing behavior
+const RouterStub = createRoutesStub([
+  {
+    Component: MyComponent,
+    path: "/conversations/:conversationId",
+  },
+]);
+
+render(<RouterStub initialEntries={["/conversations/123"]} />);
+```
+
+## Common Patterns
+
+### Combining with `QueryClientProvider`
+
+Many components need both routing and TanStack Query context:
+
+```typescript
+import { createRoutesStub } from "react-router";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: { retry: false },
+  },
+});
+
+const RouterStub = createRoutesStub([
+  {
+    Component: MyComponent,
+    path: "/",
+  },
+]);
+
+render(<RouterStub />, {
+  wrapper: ({ children }) => (
+    <QueryClientProvider client={queryClient}>
+      {children}
+    </QueryClientProvider>
+  ),
+});
+```
+
+### Testing navigation behavior
+
+Verify that user interactions trigger the expected navigation:
+
+```typescript
+import { createRoutesStub } from "react-router";
+import { screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+
+const RouterStub = createRoutesStub([
+  {
+    Component: HomeScreen,
+    path: "/",
+  },
+  {
+    Component: () => <div data-testid="settings-screen" />,
+    path: "/settings",
+  },
+]);
+
+render(<RouterStub initialEntries={["/"]} />);
+
+const user = userEvent.setup();
+await user.click(screen.getByRole("link", { name: /settings/i }));
+
+expect(screen.getByTestId("settings-screen")).toBeInTheDocument();
+```
+
+## See Also
+
+### Codebase Examples
+
+- [settings.test.tsx](__tests__/routes/settings.test.tsx) - `createRoutesStub` with nested routes and loaders
+- [home-screen.test.tsx](__tests__/routes/home-screen.test.tsx) - `createRoutesStub` with navigation testing
+- [chat-interface.test.tsx](__tests__/components/chat/chat-interface.test.tsx) - `MemoryRouter` usage
+
+### Official Documentation
+
+- [React Router Testing Guide](https://reactrouter.com/start/framework/testing) - Official guide on testing with `createRoutesStub`
+- [MemoryRouter API](https://reactrouter.com/api/declarative-routers/MemoryRouter) - API reference for `MemoryRouter`