test(FN-4308): complete Step 4 — add card and modal aggregation regressions

Fusion-Task-Id: FN-4308
Fusion-Task-Lineage: d0a46a8f-2245-4ddc-99b4-06d78e87db16

packages/dashboard/app/components/__tests__/TaskCard.test.tsx

@@ -25,6 +25,11 @@ vi.mock("../ProviderIcon", () => ({
   ProviderIcon: ({ provider }: { provider: string }) => <span data-testid={`provider-icon-${provider}`} />,
 }));
 
+const useTaskDiffStatsMock = vi.fn(() => ({ stats: null, loading: false }));
+vi.mock("../../hooks/useTaskDiffStats", () => ({
+  useTaskDiffStats: (...args: any[]) => useTaskDiffStatsMock(...args),
+}));
+
 // Mock the api module
 vi.mock("../../api", () => ({
   fetchTaskDetail: vi.fn(),
@@ -78,6 +83,7 @@ const highFanout = {
 
 afterEach(() => {
   vi.useRealTimers();
+  useTaskDiffStatsMock.mockReturnValue({ stats: null, loading: false });
 });
 
 describe("TaskCard", () => {
@@ -1453,6 +1459,35 @@ describe("TaskCard", () => {
     expect(container.querySelector(".card-time-indicator")).not.toBeNull();
   });
 
+  it("prefers done-task /diff filesChanged over mergeDetails.filesChanged", () => {
+    useTaskDiffStatsMock.mockReturnValue({
+      stats: { filesChanged: 4, additions: 10, deletions: 2 },
+      loading: false,
+    });
+
+    render(
+      <TaskCard
+        task={makeTask({
+          column: "done",
+          mergeDetails: {
+            commitSha: "abc123",
+            filesChanged: 1,
+            insertions: 10,
+            deletions: 2,
+            mergedAt: "2026-04-25T15:00:00.000Z",
+            mergeConfirmed: true,
+          },
+        })}
+        onOpenDetail={noop}
+        addToast={noop}
+        onOpenDetailWithTab={vi.fn()}
+      />,
+    );
+
+    expect(screen.getByRole("button", { name: "4 files changed" })).toBeDefined();
+    expect(screen.queryByRole("button", { name: "1 file changed" })).toBeNull();
+  });
+
   it("renders files-changed metadata and timer chip in footer row", () => {
     const { container } = render(
       <TaskCard

packages/dashboard/app/components/__tests__/TaskChangesTab.test.tsx

@@ -246,6 +246,35 @@ describe("TaskChangesTab — commit-backed (done tasks)", () => {
     expect(mockFetchTaskDiff).toHaveBeenCalledWith("FN-001", undefined, undefined);
   });
 
+  it("renders aggregated done-task file union and matching header count", async () => {
+    mockFetchTaskDiff.mockResolvedValue({
+      files: [
+        { path: "a.ts", status: "modified", additions: 1, deletions: 0, patch: "@@" },
+        { path: "b.ts", status: "added", additions: 2, deletions: 0, patch: "@@" },
+        { path: "c.ts", status: "deleted", additions: 0, deletions: 1, patch: "@@" },
+        { path: "d.ts", status: "modified", additions: 3, deletions: 2, patch: "@@" },
+      ],
+      stats: { filesChanged: 4, additions: 6, deletions: 3 },
+    });
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("Files Changed (4)")).toBeTruthy();
+    });
+    expect(screen.getByText("a.ts")).toBeTruthy();
+    expect(screen.getByText("b.ts")).toBeTruthy();
+    expect(screen.getByText("c.ts")).toBeTruthy();
+    expect(screen.getByText("d.ts")).toBeTruthy();
+  });
+
   it("shows commit metadata for done task", async () => {
     mockFetchTaskDiff.mockResolvedValue(DONE_TASK_DIFF);
 

packages/dashboard/app/hooks/useTaskDiffStats.ts

@@ -70,9 +70,9 @@ export function __test_clearDiffStatsCache(): void {
  * Fetches diff stats for a task's Changes tab.
  *
  * For active worktree-backed tasks, this keeps the TaskCard count aligned with
- * the Changes tab. For done tasks, it uses the same endpoint so the card does
+ * the Changes tab. For done tasks, it always uses `/api/tasks/:id/diff`, whose
- * not fall back to `mergeDetails.filesChanged`, which is computed at merge time
+ * server-side aggregation is the canonical source (including multi-commit
- * and can differ from the endpoint's count.
+ * lineage unions), instead of trusting `mergeDetails.filesChanged`.
  *
  * @param taskId - Task identifier
  * @param column - Current task column