fix(GRO-2094): instrument bootstrap with global error + ErrorBoundary

The bundle at /login was executing but the React tree never painted —
no console errors, no fallback UI, just an empty <div id='root'>.
Add three layers of defense so a future failure of this shape is
captured instead of being silently swallowed:

  1. window 'error' and 'unhandledrejection' listeners in main.tsx,
     printing structured context to console.error so Playwright
     sees the failure in the console log even if React unmounts
     the tree.

  2. A top-level <ErrorBoundary> in main.tsx that renders the
     actual exception (name, message, stack) inside the DOM
     instead of leaving <div id='root'> empty. The boundary also
     logs to console.error via componentDidCatch.

  3. New tests for the ErrorBoundary (renders children, surfaces
     thrown errors visibly) and two new UAT_PLAYBOOK test cases
     (TC-WEB-5.1.6 / 5.1.7) that explicitly assert the
     'never-blank-root' invariant on UAT.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

UAT_PLAYBOOK.md

@@ -54,6 +54,8 @@ export const { signIn, signOut, useSession, changePassword } = authClient;
 | TC-WEB-5.1.3 | Logout | Click logout button | Session cleared, redirected to login page |
 | TC-WEB-5.1.4 | Session indicator | After successful login | User info/initials visible in UI indicating active session |
 | TC-WEB-5.1.5 | Unauthenticated `/login` renders the form (GRO-2011) | In a private/incognito window with no session cookie, navigate to UAT `/login` | React root mounts; the GroomBook sign-in card with the OIDC button is visible. Network tab shows `/api/auth/get-session` 200, `/api/setup/status` 200, and the login form is rendered (NOT a blank white viewport). |
+| TC-WEB-5.1.6 | Swallowed render error surfaces in DOM (GRO-2094) | Trigger a render-time exception in the React tree (e.g. via temporary throw in a child component on a test build) and load `/login` in a clean context | Either the login form renders normally (happy path) OR the top-level `ErrorBoundary` testid `error-boundary` is visible with a populated `error-boundary-message` pre block showing the exception name/message/stack. **NEVER** a blank `<div id="root">` with no error indicator. Browser console must contain either zero render errors or a `[ErrorBoundary]` line plus the raw exception. |
+| TC-WEB-5.1.7 | Global `error` and `unhandledrejection` listeners are wired (GRO-2094) | In a clean browser context, load `/login`, then trigger `setTimeout(() => { throw new Error("synthetic") }, 0)` from the console and `Promise.reject(new Error("synthetic-promise"))` | Browser console shows `[window.error]` and `[unhandledrejection]` log lines with the thrown values. Confirms global listeners are active in production. |
 
 ### 5.2 Authentication — VITE_API_URL Set
 

src/ErrorBoundary.tsx

@@ -0,0 +1,77 @@
+import { Component } from "react";
+import type { ErrorInfo, ReactNode } from "react";
+
+interface ErrorBoundaryProps {
+  children: ReactNode;
+}
+
+interface ErrorBoundaryState {
+  error: Error | null;
+}
+
+/**
+ * Top-level ErrorBoundary — renders the error visibly so the actual exception
+ * appears in the DOM (and therefore in the Playwright snapshot) instead of
+ * React 18+ unmounting the entire tree to a blank `<div id="root">`.
+ *
+ * Background: GRO-2094. The bundle was executing but never painting, with
+ * the failure swallowed. Surfacing the error here is the first step; the
+ * real fix is in the underlying component that threw.
+ */
+export class ErrorBoundary extends Component<ErrorBoundaryProps, ErrorBoundaryState> {
+  state: ErrorBoundaryState = { error: null };
+
+  static getDerivedStateFromError(error: Error): ErrorBoundaryState {
+    return { error };
+  }
+
+  componentDidCatch(error: Error, info: ErrorInfo): void {
+    // Also surface to the console — this is what the test harness greps for.
+    // eslint-disable-next-line no-console
+    console.error("[ErrorBoundary] Uncaught render error:", error, info);
+  }
+
+  render() {
+    if (this.state.error) {
+      const err = this.state.error;
+      return (
+        <div
+          data-testid="error-boundary"
+          style={{
+            padding: "2rem",
+            fontFamily: "ui-monospace, SFMono-Regular, Menlo, monospace",
+            color: "#7f1d1d",
+            background: "#fef2f2",
+            minHeight: "100vh",
+            boxSizing: "border-box",
+          }}
+        >
+          <h1 style={{ fontSize: 18, margin: "0 0 0.5rem" }}>Something went wrong</h1>
+          <p style={{ margin: "0 0 1rem", color: "#991b1b" }}>
+            The app failed to render. The full error is shown below — please share this
+            output when reporting the bug.
+          </p>
+          <pre
+            data-testid="error-boundary-message"
+            style={{
+              whiteSpace: "pre-wrap",
+              wordBreak: "break-word",
+              background: "#fff",
+              border: "1px solid #fecaca",
+              borderRadius: 6,
+              padding: "0.75rem 1rem",
+              margin: 0,
+              fontSize: 13,
+              lineHeight: 1.4,
+            }}
+          >
+            {err.name}: {err.message}
+            {"\n\n"}
+            {err.stack ?? "(no stack)"}
+          </pre>
+        </div>
+      );
+    }
+    return this.props.children;
+  }
+}

src/__tests__/ErrorBoundary.test.tsx

@@ -0,0 +1,54 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { render, screen, cleanup } from "@testing-library/react";
+import { ErrorBoundary } from "../ErrorBoundary";
+
+function ThrowingChild(): never {
+  throw new Error("synthetic render-time failure for GRO-2094");
+}
+
+function GoodChild() {
+  return <div data-testid="good-child">ok</div>;
+}
+
+describe("ErrorBoundary (GRO-2094)", () => {
+  let errorSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    // React 18+ logs caught render errors to console.error via React's own
+    // instrumentation; suppress it so test output is clean but capture it
+    // for an assertion below.
+    errorSpy = vi.spyOn(console, "error").mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    errorSpy.mockRestore();
+    cleanup();
+  });
+
+  it("renders children when nothing throws", () => {
+    render(
+      <ErrorBoundary>
+        <GoodChild />
+      </ErrorBoundary>
+    );
+    expect(screen.getByTestId("good-child")).toBeInTheDocument();
+    expect(screen.queryByTestId("error-boundary")).not.toBeInTheDocument();
+  });
+
+  it("renders the error visibly when a child throws during render", () => {
+    render(
+      <ErrorBoundary>
+        <ThrowingChild />
+      </ErrorBoundary>
+    );
+
+    const fallback = screen.getByTestId("error-boundary");
+    expect(fallback).toBeInTheDocument();
+    const message = screen.getByTestId("error-boundary-message");
+    // The actual exception is shown — no more silent blank root.
+    expect(message.textContent).toContain("synthetic render-time failure for GRO-2094");
+    // The boundary also calls console.error so it shows up in the Playwright
+    // console log even if the DOM-rendered fallback is somehow missed.
+    expect(errorSpy).toHaveBeenCalled();
+  });
+});

src/main.tsx

@@ -2,9 +2,41 @@ import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
 import { BrowserRouter } from "react-router-dom";
 import { App } from "./App.js";
+import { ErrorBoundary } from "./ErrorBoundary.js";
 import { installDevFetchInterceptor } from "./lib/devFetch.js";
 import "./index.css";
 
+// --------------------------------------------------------------------
+// Global error capture (GRO-2094).
+//
+// Symptom: React root stays empty at /login — bundle parses, no console
+// errors, no error boundary fallback. Some failure is being swallowed
+// before it reaches React's commit phase. These listeners make sure any
+// thrown error or unhandled promise rejection is at least visible in
+// the console (and in the Playwright network/console log) instead of
+// vanishing into the void.
+// --------------------------------------------------------------------
+function reportGlobalError(kind: string, payload: unknown): void {
+  // eslint-disable-next-line no-console
+  console.error(`[${kind}]`, payload);
+}
+
+window.addEventListener("error", (event) => {
+  reportGlobalError("window.error", {
+    message: event.message,
+    filename: event.filename,
+    lineno: event.lineno,
+    colno: event.colno,
+    error: event.error,
+  });
+});
+
+window.addEventListener("unhandledrejection", (event) => {
+  reportGlobalError("unhandledrejection", {
+    reason: event.reason,
+  });
+});
+
 installDevFetchInterceptor();
 
 const root = document.getElementById("root");
@@ -12,8 +44,10 @@ if (!root) throw new Error("Root element not found");
 
 createRoot(root).render(
   <StrictMode>
-    <BrowserRouter>
+    <ErrorBoundary>
-      <App />
+      <BrowserRouter>
-    </BrowserRouter>
+        <App />
+      </BrowserRouter>
+    </ErrorBoundary>
   </StrictMode>
 );