test(backlog): add 3-pane integration tests for click-cascade flow

Covers: empty states, PBI→stories, story→tasks, cascade-reset,
isSelected highlight. localStorage mocked for sort-mode persistence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

__tests__/components/backlog/integration.test.tsx

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+// @vitest-environment jsdom
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { render, screen, fireEvent } from '@testing-library/react'
+import { useSelectionStore } from '@/stores/selection-store'
+import { useBacklogStore } from '@/stores/backlog-store'
+
+// Mock next/navigation
+const mockPush = vi.fn()
+vi.mock('next/navigation', () => ({ useRouter: () => ({ push: mockPush }) }))
+
+// localStorage mock for StoryPanel sort mode persistence
+const localStorageMock = (() => {
+  let store: Record<string, string> = {}
+  return {
+    getItem: (k: string) => store[k] ?? null,
+    setItem: (k: string, v: string) => { store[k] = v },
+    removeItem: (k: string) => { delete store[k] },
+    clear: () => { store = {} },
+  }
+})()
+Object.defineProperty(globalThis, 'localStorage', { value: localStorageMock, writable: true })
+
+// Mock server actions
+vi.mock('@/actions/stories', () => ({
+  reorderStoriesAction: vi.fn().mockResolvedValue({ success: true }),
+  reorderPbisAction: vi.fn().mockResolvedValue({ success: true }),
+  updatePbiPriorityAction: vi.fn().mockResolvedValue({ success: true }),
+}))
+vi.mock('@/actions/pbis', () => ({ deletePbiAction: vi.fn().mockResolvedValue({ success: true }) }))
+vi.mock('@/actions/tasks', () => ({ reorderTasksAction: vi.fn().mockResolvedValue({ success: true }) }))
+vi.mock('sonner', () => ({ toast: { error: vi.fn(), success: vi.fn() } }))
+
+// Mock dnd-kit
+vi.mock('@dnd-kit/core', () => ({
+  DndContext: ({ children }: { children: React.ReactNode }) => <>{children}</>,
+  PointerSensor: class {},
+  KeyboardSensor: class {},
+  useSensor: vi.fn(),
+  useSensors: vi.fn(() => []),
+  closestCenter: vi.fn(),
+  DragOverlay: () => null,
+}))
+vi.mock('@dnd-kit/sortable', () => ({
+  SortableContext: ({ children }: { children: React.ReactNode }) => <>{children}</>,
+  useSortable: () => ({
+    attributes: {}, listeners: {}, setNodeRef: vi.fn(),
+    transform: null, transition: undefined, isDragging: false,
+  }),
+  verticalListSortingStrategy: {},
+  rectSortingStrategy: {},
+  sortableKeyboardCoordinates: {},
+  arrayMove: (arr: unknown[]) => arr,
+}))
+vi.mock('@dnd-kit/utilities', () => ({ CSS: { Transform: { toString: () => '' } } }))
+
+import { StoryPanel } from '@/components/backlog/story-panel'
+import { TaskPanel } from '@/components/backlog/task-panel'
+
+const PRODUCT_ID = 'prod-1'
+const PBI_ID = 'pbi-1'
+const ALT_PBI_ID = 'pbi-2'
+const STORY_ID = 'story-1'
+
+const STORIES = [
+  { id: STORY_ID, code: 'ST-1', title: 'Eerste story', description: null, acceptance_criteria: null, priority: 2, status: 'OPEN', pbi_id: PBI_ID, created_at: new Date() },
+]
+const TASKS = [
+  { id: 'task-1', title: 'Eerste taak', description: null, priority: 2, status: 'TO_DO', sort_order: 1, story_id: STORY_ID, created_at: new Date() },
+]
+
+function resetStores() {
+  useSelectionStore.setState({ selectedPbiId: null, selectedStoryId: null })
+  useBacklogStore.setState({
+    pbis: [],
+    storiesByPbi: { [PBI_ID]: STORIES },
+    tasksByStory: { [STORY_ID]: TASKS },
+  })
+}
+
+describe('Backlog 3-pane integration', () => {
+  beforeEach(() => {
+    mockPush.mockClear()
+    resetStores()
+  })
+
+  it('StoryPanel shows empty state when no PBI selected', () => {
+    render(<StoryPanel productId={PRODUCT_ID} isDemo={false} />)
+    expect(screen.getByText('Selecteer een PBI om de stories te bekijken.')).toBeTruthy()
+  })
+
+  it('StoryPanel shows stories when PBI is selected', () => {
+    useSelectionStore.setState({ selectedPbiId: PBI_ID, selectedStoryId: null })
+    render(<StoryPanel productId={PRODUCT_ID} isDemo={false} />)
+    expect(screen.getByText('Eerste story')).toBeTruthy()
+  })
+
+  it('clicking a story dispatches selectStory to the store', () => {
+    useSelectionStore.setState({ selectedPbiId: PBI_ID, selectedStoryId: null })
+    render(<StoryPanel productId={PRODUCT_ID} isDemo={false} />)
+    fireEvent.click(screen.getByText('Eerste story'))
+    expect(useSelectionStore.getState().selectedStoryId).toBe(STORY_ID)
+  })
+
+  it('cascade-reset: selecting different PBI clears selectedStoryId', () => {
+    useSelectionStore.setState({ selectedPbiId: PBI_ID, selectedStoryId: STORY_ID })
+    useSelectionStore.getState().selectPbi(ALT_PBI_ID)
+    expect(useSelectionStore.getState().selectedStoryId).toBeNull()
+  })
+
+  it('TaskPanel shows tasks after story is selected', () => {
+    useSelectionStore.setState({ selectedPbiId: PBI_ID, selectedStoryId: STORY_ID })
+    render(<TaskPanel productId={PRODUCT_ID} isDemo={false} closePath={`/products/${PRODUCT_ID}`} />)
+    expect(screen.getByText('Eerste taak')).toBeTruthy()
+  })
+
+  it('TaskPanel shows empty state after cascade-reset', () => {
+    useSelectionStore.setState({ selectedPbiId: PBI_ID, selectedStoryId: STORY_ID })
+    render(<TaskPanel productId={PRODUCT_ID} isDemo={false} closePath={`/products/${PRODUCT_ID}`} />)
+    // Reset via selectPbi
+    useSelectionStore.getState().selectPbi(ALT_PBI_ID)
+    // Re-render reflects new store state
+    render(<TaskPanel productId={PRODUCT_ID} isDemo={false} closePath={`/products/${PRODUCT_ID}`} />)
+    expect(screen.getAllByText('Selecteer een story om de taken te bekijken.').length).toBeGreaterThan(0)
+  })
+
+  it('selected story card has isSelected highlight class applied', () => {
+    useSelectionStore.setState({ selectedPbiId: PBI_ID, selectedStoryId: STORY_ID })
+    const { container } = render(<StoryPanel productId={PRODUCT_ID} isDemo={false} />)
+    // bg-primary-container is applied when isSelected
+    const selected = container.querySelector('.bg-primary-container')
+    expect(selected).toBeTruthy()
+  })
+})