fix(FN-6117): correct stacked branch diff counts

Filter active task diff displays to commits attributed to the current task when a branch range includes foreign task commits. This keeps task cards and Changes tabs from showing inherited branch stacks as task-owned file changes.

Fusion-Task-Id: FN-6117

.changeset/fn-6117-stacked-branch-diff-count.md

@@ -0,0 +1,5 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix task changed-file counts for stacked or cherry-equivalent task branches by filtering active review diffs to commits attributed to the current task.

packages/dashboard/src/__tests__/routes-file-diffs.test.ts

@@ -109,6 +109,11 @@ async function requestFileDiffs(app: Parameters<typeof import("../test-request.j
   return get(app, `/api/tasks/${taskId}/file-diffs`);
 }
 
+async function requestTaskDiff(app: Parameters<typeof import("../test-request.js").get>[0], taskId = "KB-651"): Promise<{ status: number; body: any }> {
+  const { get } = await import("../test-request.js");
+  return get(app, `/api/tasks/${taskId}/diff`);
+}
+
 describe("GET /api/tasks/:id/file-diffs", () => {
   beforeEach(() => {
     vi.clearAllMocks();
@@ -266,6 +271,69 @@ describe("GET /api/tasks/:id/file-diffs", () => {
       rmSync(repoDir, { recursive: true, force: true });
     }
   }, 15_000);
+
+  it("restricts active stacked branch diffs to commits attributed to the task", async () => {
+    const repoDir = mkdtempSync(join(tmpdir(), "fn-active-stacked-branch-diff-"));
+
+    try {
+      execFileSync("git", ["init", "-b", "main", repoDir], { stdio: "pipe" });
+      execFileSync("git", ["-C", repoDir, "config", "user.email", "stacked@example.com"], { stdio: "pipe" });
+      execFileSync("git", ["-C", repoDir, "config", "user.name", "Stacked Test"], { stdio: "pipe" });
+
+      writeFileSync(join(repoDir, "README.md"), "# stacked\n");
+      execFileSync("git", ["-C", repoDir, "add", "README.md"], { stdio: "pipe" });
+      execFileSync("git", ["-C", repoDir, "commit", "-m", "initial"], { stdio: "pipe" });
+      const forkPoint = execFileSync("git", ["-C", repoDir, "rev-parse", "HEAD"], { encoding: "utf-8", stdio: "pipe" }).trim();
+
+      writeFileSync(join(repoDir, "foreign-upstream.ts"), "upstream\n");
+      execFileSync("git", ["-C", repoDir, "add", "foreign-upstream.ts"], { stdio: "pipe" });
+      execFileSync("git", ["-C", repoDir, "commit", "-m", "FN-6118: upstream landed"], { stdio: "pipe" });
+      const recordedBase = execFileSync("git", ["-C", repoDir, "rev-parse", "HEAD"], { encoding: "utf-8", stdio: "pipe" }).trim();
+
+      execFileSync("git", ["-C", repoDir, "checkout", "-b", "fusion/fn-6117", forkPoint], { stdio: "pipe" });
+      writeFileSync(join(repoDir, "foreign-copy.ts"), "copied foreign work\n");
+      execFileSync("git", ["-C", repoDir, "add", "foreign-copy.ts"], { stdio: "pipe" });
+      execFileSync("git", ["-C", repoDir, "commit", "-m", "FN-6118: upstream landed"], { stdio: "pipe" });
+
+      writeFileSync(join(repoDir, "own-a.ts"), "own a\n");
+      execFileSync("git", ["-C", repoDir, "add", "own-a.ts"], { stdio: "pipe" });
+      execFileSync("git", ["-C", repoDir, "commit", "-m", "test(FN-6117): add own a"], { stdio: "pipe" });
+
+      writeFileSync(join(repoDir, "own-b.ts"), "own b\n");
+      execFileSync("git", ["-C", repoDir, "add", "own-b.ts"], { stdio: "pipe" });
+      execFileSync("git", ["-C", repoDir, "commit", "-m", "feat(FN-6117): add own b"], { stdio: "pipe" });
+
+      const rawBranchFiles = execFileSync("git", ["-C", repoDir, "diff", "--name-only", "main...fusion/fn-6117"], {
+        encoding: "utf-8",
+        stdio: "pipe",
+      }).trim().split("\n").filter(Boolean);
+      expect(rawBranchFiles).toHaveLength(3);
+
+      const store = new RepoBackedStore(repoDir);
+      store.addTask(createTask({
+        id: "FN-6117",
+        column: "in-review",
+        worktree: repoDir,
+        branch: "fusion/fn-6117",
+        baseBranch: "main",
+        baseCommitSha: recordedBase,
+      }));
+
+      const app = createServer(store as any);
+      const diffResponse = await requestTaskDiff(app, "FN-6117");
+      const fileDiffsResponse = await requestFileDiffs(app, "FN-6117");
+
+      expect(diffResponse.status).toBe(200);
+      expect(diffResponse.body.stats.filesChanged).toBe(2);
+      expect(diffResponse.body.files.map((file: { path: string }) => file.path).sort()).toEqual(["own-a.ts", "own-b.ts"]);
+      expect(diffResponse.body.files.map((file: { path: string }) => file.path)).not.toContain("foreign-copy.ts");
+
+      expect(fileDiffsResponse.status).toBe(200);
+      expect(fileDiffsResponse.body.map((file: { path: string }) => file.path).sort()).toEqual(["own-a.ts", "own-b.ts"]);
+    } finally {
+      rmSync(repoDir, { recursive: true, force: true });
+    }
+  }, 15_000);
 });
 
 describe("resolveDiffBase", () => {

packages/dashboard/src/routes/register-session-diff-routes.ts

@@ -313,6 +313,41 @@ function parseNameStatusLine(line: string): { statusCode: string; path: string;
   return oldPath ? { statusCode, path, oldPath } : { statusCode, path };
 }
 
+async function restrictActiveCommittedFilesToOwnTask<T>(
+  fileMap: Map<string, T>,
+  input: {
+    taskId: string;
+    diffBase?: string;
+    worktreePath: string;
+    runGit: (args: string[]) => Promise<string>;
+  },
+): Promise<void> {
+  if (!input.diffBase || fileMap.size === 0) return;
+
+  try {
+    const attribution = await filterFilesToOwnTaskCommits({
+      worktreePath: input.worktreePath,
+      baseRef: input.diffBase,
+      taskId: input.taskId,
+      runGit: input.runGit,
+    });
+
+    if (attribution.foreignCommitCount === 0 || attribution.ownCommitShas.length === 0 || attribution.files.length === 0) {
+      return;
+    }
+
+    const ownFiles = new Set(attribution.files);
+    for (const filePath of fileMap.keys()) {
+      if (!ownFiles.has(filePath)) {
+        fileMap.delete(filePath);
+      }
+    }
+  } catch {
+    // Display-only attribution. If git metadata is unavailable, preserve the
+    // existing broad diff rather than hiding task files.
+  }
+}
+
 async function collectDoneRangeFiles(range: string, rootDir: string): Promise<AggregatedDoneTaskFile[]> {
   const nameStatus = (await runGitCommand(["diff", "--name-status", "-M", range], rootDir, 10000)).trim();
   const files: AggregatedDoneTaskFile[] = [];
@@ -892,6 +927,13 @@ export function registerSessionDiffRoutes(router: Router, deps: SessionDiffRoute
         }
       }
 
+      await restrictActiveCommittedFilesToOwnTask(fileMap, {
+        taskId: task.id,
+        diffBase,
+        worktreePath: cwd,
+        runGit: (args) => runGitCommand(args, cwd, 10000),
+      });
+
       try {
         const stagedOutput = (await runGitCommand(["diff", "--cached", "--name-status", "-M"], cwd, 10000)).trim();
         for (const line of stagedOutput.split("\n").filter(Boolean)) {
@@ -1131,6 +1173,13 @@ export function registerSessionDiffRoutes(router: Router, deps: SessionDiffRoute
         }
       }
 
+      await restrictActiveCommittedFilesToOwnTask(fileMap, {
+        taskId: task.id,
+        diffBase,
+        worktreePath: cwd,
+        runGit: (args) => runGitCommand(args, cwd, 5000),
+      });
+
       try {
         const stagedOutput = (await runGitCommand(["diff", "--cached", "--name-status", "-M"], cwd, 5000)).trim();
         for (const line of stagedOutput.split("\n").filter(Boolean)) {