diff --git a/apps/pwa/src/components/EventDetailPopover.test.tsx b/apps/pwa/src/components/EventDetailPopover.test.tsx
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+/**
+ * EventDetailPopover tests — Task 1 (TDD RED → GREEN)
+ *
+ * Guards:
+ *   - T-02e-01: Title containing HTML-looking strings renders as escaped text (XSS guard)
+ *   - Escape key closes the popover and clears openEventId
+ *   - All event fields (title, location, description, calendar name) render as text
+ *   - Close button has aria-label="Close"
+ *   - Backdrop click closes the popover
+ */
+
+import 'temporal-polyfill/global'
+import React from 'react'
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { render, screen, fireEvent } from '@testing-library/react'
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+
+// window.matchMedia polyfilled in test-setup.ts
+
+// ── Module mocks ──────────────────────────────────────────────────────────────
+
+const mockSetOpenEventId = vi.fn()
+let mockOpenEventId: string | null = null
+
+vi.mock('../store/calendarStore.js', () => ({
+  useCalendarStore: vi.fn(() => ({
+    openEventId: mockOpenEventId,
+    setOpenEventId: mockSetOpenEventId,
+  })),
+}))
+
+// ── Fixtures ───────────────────────────────────────────────────────────────────
+
+const TIMED_OCCURRENCE = {
+  id: 'test-uid::2026-06-15T10:00:00',
+  uid: 'test-uid',
+  calendarId: 1,
+  calendarName: 'My Calendar',
+  ownerUserId: 1,
+  color: '#4A90D9',
+  isShared: false,
+  title: 'Team Standup',
+  start: '2026-06-15T10:00:00-04:00[America/New_York]',
+  end: '2026-06-15T10:30:00-04:00[America/New_York]',
+  allDay: false,
+  location: 'Conference Room B',
+  description: 'Daily team sync meeting',
+}
+
+const OCCURRENCE_WITH_HTML = {
+  ...TIMED_OCCURRENCE,
+  id: 'xss-uid::2026-06-15T10:00:00',
+  uid: 'xss-uid',
+  title: '<script>alert("xss")</script>Team Meeting',
+  description: '<b>Bold</b> description',
+  location: '<img src=x onerror=alert(1)>Room',
+}
+
+const ALLDAY_OCCURRENCE = {
+  id: 'allday-uid::2026-06-20',
+  uid: 'allday-uid',
+  calendarId: 2,
+  calendarName: 'Shared Calendar',
+  ownerUserId: 1,
+  color: '#F25C7A',
+  isShared: true,
+  title: 'Birthday Party',
+  start: '2026-06-20',
+  end: '2026-06-20',
+  allDay: true,
+  location: null,
+  description: null,
+}
+
+// ── Import component (after mocks are declared) ───────────────────────────────
+
+import { EventDetailPopover } from './EventDetailPopover.js'
+import { useCalendarStore } from '../store/calendarStore.js'
+
+// ── Helpers ───────────────────────────────────────────────────────────────────
+
+function renderPopover(occurrence = TIMED_OCCURRENCE) {
+  mockOpenEventId = occurrence.id
+  ;(useCalendarStore as ReturnType<typeof vi.fn>).mockImplementation(() => ({
+    openEventId: mockOpenEventId,
+    setOpenEventId: mockSetOpenEventId,
+  }))
+
+  const client = new QueryClient({
+    defaultOptions: { queries: { retry: false, staleTime: Infinity } },
+  })
+  // Pre-populate the events cache so EventDetailPopover can resolve by id
+  client.setQueryData(['events'], { occurrences: [occurrence] })
+
+  return render(
+    <QueryClientProvider client={client}>
+      <EventDetailPopover />
+    </QueryClientProvider>,
+  )
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────────────
+
+describe('EventDetailPopover', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    mockOpenEventId = null
+  })
+
+  it('renders the event title as a heading', () => {
+    renderPopover(TIMED_OCCURRENCE)
+    expect(screen.getByRole('heading')).toHaveTextContent('Team Standup')
+  })
+
+  it('renders location when present', () => {
+    renderPopover(TIMED_OCCURRENCE)
+    expect(screen.getByText(/Conference Room B/)).toBeDefined()
+  })
+
+  it('renders description when present', () => {
+    renderPopover(TIMED_OCCURRENCE)
+    expect(screen.getByText(/Daily team sync meeting/)).toBeDefined()
+  })
+
+  it('renders calendar name in footer', () => {
+    renderPopover(TIMED_OCCURRENCE)
+    expect(screen.getByText(/My Calendar/)).toBeDefined()
+  })
+
+  it('renders an all-day event without crashing', () => {
+    renderPopover(ALLDAY_OCCURRENCE)
+    expect(screen.getByRole('heading')).toHaveTextContent('Birthday Party')
+  })
+
+  it('close button has aria-label="Close"', () => {
+    renderPopover(TIMED_OCCURRENCE)
+    expect(screen.getByLabelText('Close')).toBeDefined()
+  })
+
+  it('pressing Escape calls setOpenEventId(null)', () => {
+    renderPopover(TIMED_OCCURRENCE)
+    fireEvent.keyDown(document, { key: 'Escape' })
+    expect(mockSetOpenEventId).toHaveBeenCalledWith(null)
+  })
+
+  it('clicking the close button calls setOpenEventId(null)', () => {
+    renderPopover(TIMED_OCCURRENCE)
+    fireEvent.click(screen.getByLabelText('Close'))
+    expect(mockSetOpenEventId).toHaveBeenCalledWith(null)
+  })
+
+  it('clicking the backdrop calls setOpenEventId(null)', () => {
+    renderPopover(TIMED_OCCURRENCE)
+    fireEvent.click(screen.getByTestId('popover-backdrop'))
+    expect(mockSetOpenEventId).toHaveBeenCalledWith(null)
+  })
+
+  it('renders nothing when openEventId is null', () => {
+    mockOpenEventId = null
+    ;(useCalendarStore as ReturnType<typeof vi.fn>).mockImplementation(() => ({
+      openEventId: null,
+      setOpenEventId: mockSetOpenEventId,
+    }))
+    const client = new QueryClient({
+      defaultOptions: { queries: { retry: false, staleTime: Infinity } },
+    })
+    const { container } = render(
+      <QueryClientProvider client={client}>
+        <EventDetailPopover />
+      </QueryClientProvider>,
+    )
+    expect(container.firstChild).toBeNull()
+  })
+
+  it('XSS guard: HTML in title renders as escaped text, not as DOM elements', () => {
+    renderPopover(OCCURRENCE_WITH_HTML)
+    const heading = screen.getByRole('heading')
+    // <script> must NOT be injected as a DOM element
+    expect(heading.innerHTML).not.toContain('<script>')
+    // The raw text including angle brackets must appear as literal text
+    expect(heading.textContent).toContain('<script>alert("xss")</script>Team Meeting')
+  })
+
+  it('XSS guard: HTML in description renders as escaped text', () => {
+    renderPopover(OCCURRENCE_WITH_HTML)
+    const descEl = screen.getByTestId('event-description')
+    // <b> must NOT be rendered as a bold element
+    expect(descEl.innerHTML).not.toContain('<b>')
+    expect(descEl.textContent).toContain('<b>Bold</b> description')
+  })
+})