feat(FN-818): use merge commit diff for done-task Changes tab

- Refactor TaskChangesTab to use merge commit diff for done/archived tasks instead of branch diff
- Add CommitDiffTab component integration for displaying merge commit changes
- Remove legacy API endpoints and tests for branch-based diff on completed tasks
- Add comprehensive test suite for TaskChangesTab covering all task lifecycle states
- Document merge commit diff behavior in dashboard README

packages/dashboard/README.md

@@ -47,7 +47,7 @@ AI-guided interactive planning for creating well-specified tasks from high-level
 - **AI-Assisted Creation Controls**: Plan, Subtask, and Refine buttons appear directly below the description textarea in all task creation surfaces (quick entry box, inline create card, and task form modal). These description-adjacent controls make AI-assisted creation and refinement feel directly associated with the text being edited. Deps, Models, and Save actions remain in the expanded controls footer.
 - **Layered Model Dropdowns**: Shared model combobox menus render in a top-level portal attached to `document.body`, so they stay above board columns and scrollable modal content instead of being clipped behind surrounding dashboard surfaces.
 - **Bulk Model Editing**: Update AI model configuration for multiple tasks at once in the list view. Select tasks via checkboxes (archived tasks excluded), then use the "Bulk Edit Models" toolbar to apply executor and/or validator model changes to all selected tasks. Selection persists in localStorage across page reloads.
-- **Task Details**: View full task specifications, agent logs, and attachments. The Agent Log tab expands to fill the full modal body height above the action bar, providing maximum vertical space for watching live agent output. The tab header shows the effective executor and validator model names resolved from task-level overrides or project/global settings fallbacks, matching the same resolution order the engine uses at runtime. The refinement modal positions the "Create Refinement Task" button adjacent to the feedback textarea alongside the character count, creating a tight input group that connects the submit action directly to the text being edited.
+- **Task Details**: View full task specifications, agent logs, and attachments. The Agent Log tab expands to fill the full modal body height above the action bar, providing maximum vertical space for watching live agent output. The tab header shows the effective executor and validator model names resolved from task-level overrides or project/global settings fallbacks, matching the same resolution order the engine uses at runtime. The refinement modal positions the "Create Refinement Task" button adjacent to the feedback textarea alongside the character count, creating a tight input group that connects the submit action directly to the text being edited. The **Changes** tab for done tasks loads the diff from the recorded merge commit (`mergeDetails.commitSha`) via `fetchCommitDiff` rather than requiring a live worktree — changes remain visible even after the worktree is cleaned up. The tab shows commit metadata (short SHA, merge commit message, merged timestamp) alongside the file-level diff with addition/deletion totals. In-progress and in-review tasks continue to use the worktree-based diff path.
 - **Changed Files Viewer**: Click a task card's "files changed" button to open a dedicated diff viewer showing only files changed in that task worktree, with per-file statuses and sidebar navigation. On mobile (≤768px), the viewer switches to a single-pane flow: the file list and diff are shown one at a time with a back button for navigation between them. The viewer always opens to the file list on mobile, and only switches to the diff view when the user taps a specific file. Pressing Escape on the diff view returns to the file list first; pressing Escape again closes the modal. Loading, error, and empty states use theme-aware styling (including light mode). Diff syntax highlighting (additions, deletions, hunks) adapts to the active theme for correct contrast. The board card file count and the changed-files viewer always agree — both use the same merge-base diff strategy, so the card never advertises files that the viewer cannot inspect
 - **GitHub Import**: Import issues directly from GitHub repositories
 - **PR Management**: Create, monitor, and merge pull requests for in-review tasks

packages/dashboard/app/components/CommitDiffTab.tsx

@@ -9,7 +9,7 @@ interface CommitDiffTabProps {
   mergeDetails?: MergeDetails;
 }
 
-interface ParsedFile {
+export interface ParsedFile {
   path: string;
   status: "added" | "modified" | "deleted" | "unknown";
   additions: number;
@@ -43,7 +43,7 @@ function getStatusLabel(status: ParsedFile["status"]): string {
   }
 }
 
-function parsePatch(rawPatch: string): ParsedFile[] {
+export function parsePatch(rawPatch: string): ParsedFile[] {
   const files: ParsedFile[] = [];
   // Split on diff boundaries, keeping the delimiter
   const parts = rawPatch.split(/(?=^diff --git )/m);

packages/dashboard/app/components/TaskChangesTab.tsx

@@ -1,21 +1,16 @@
 import { useState, useEffect, useCallback } from "react";
-import { FileCode, ChevronDown, ChevronRight, AlertCircle } from "lucide-react";
-import { fetchTaskDiff, type TaskDiff } from "../api";
+import { FileCode, ChevronDown, ChevronRight, AlertCircle, GitCommit } from "lucide-react";
+import type { MergeDetails, Column } from "@fusion/core";
+import { fetchTaskDiff, fetchCommitDiff, type TaskDiff } from "../api";
+import { parsePatch, type ParsedFile } from "./CommitDiffTab";
 import { highlightDiff } from "../utils/highlightDiff";
 
 interface TaskChangesTabProps {
   taskId: string;
   worktree?: string;
   projectId?: string;
-}
-
-function getFileStatus(file: string, patch: string): "added" | "modified" | "deleted" | "unknown" {
-  if (patch.includes("diff --git")) {
-    if (patch.includes("new file mode")) return "added";
-    if (patch.includes("deleted file mode")) return "deleted";
-    return "modified";
-  }
-  return "unknown";
+  column?: Column;
+  mergeDetails?: MergeDetails;
 }
 
 function getStatusColor(status: "added" | "modified" | "deleted" | "unknown"): string {
@@ -31,19 +26,79 @@ function getStatusColor(status: "added" | "modified" | "deleted" | "unknown"): s
   }
 }
 
-function formatFileSize(bytes: number): string {
-  if (bytes < 1024) return `${bytes} B`;
-  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
-  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+function getStatusLabel(status: "added" | "modified" | "deleted" | "unknown"): string {
+  switch (status) {
+    case "added":
+      return "A";
+    case "deleted":
+      return "D";
+    case "modified":
+      return "M";
+    default:
+      return "?";
+  }
 }
 
-export function TaskChangesTab({ taskId, worktree, projectId }: TaskChangesTabProps) {
-  const [diffData, setDiffData] = useState<TaskDiff | null>(null);
+/** Normalized file entry used by both worktree-backed and commit-backed paths */
+interface NormalizedFile {
+  path: string;
+  status: "added" | "modified" | "deleted" | "unknown";
+  additions: number;
+  deletions: number;
+  patch: string;
+}
+
+/**
+ * TaskChangesTab displays file-level diffs for a task.
+ *
+ * For in-progress/in-review tasks it loads the diff from the live worktree.
+ * For done tasks with a recorded merge commit (mergeDetails.commitSha) it loads
+ * the diff from git history instead, so changes remain visible even after the
+ * worktree is cleaned up.
+ */
+export function TaskChangesTab({ taskId, worktree, projectId, column, mergeDetails }: TaskChangesTabProps) {
+  const [files, setFiles] = useState<NormalizedFile[]>([]);
+  const [stats, setStats] = useState<{ filesChanged: number; additions: number; deletions: number }>({ filesChanged: 0, additions: 0, deletions: 0 });
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
   const [expandedFiles, setExpandedFiles] = useState<Set<string>>(new Set());
 
+  const commitSha = column === "done" ? mergeDetails?.commitSha : undefined;
+  const useCommitDiff = !!commitSha;
+
   const loadDiff = useCallback(async () => {
+    // Done task with merge commit → use commit-backed diff
+    if (useCommitDiff) {
+      try {
+        setLoading(true);
+        setError(null);
+        const data = await fetchCommitDiff(commitSha);
+        const parsed = parsePatch(data.patch || "");
+        const normalized: NormalizedFile[] = parsed.map((f) => ({
+          path: f.path,
+          status: f.status,
+          additions: f.additions,
+          deletions: f.deletions,
+          patch: f.patch,
+        }));
+        setFiles(normalized);
+        setStats({
+          filesChanged: mergeDetails?.filesChanged ?? normalized.length,
+          additions: mergeDetails?.insertions ?? normalized.reduce((s, f) => s + f.additions, 0),
+          deletions: mergeDetails?.deletions ?? normalized.reduce((s, f) => s + f.deletions, 0),
+        });
+        if (normalized.length > 0) {
+          setExpandedFiles(new Set([normalized[0].path]));
+        }
+      } catch (err: any) {
+        setError(err.message || "Failed to load commit diff");
+      } finally {
+        setLoading(false);
+      }
+      return;
+    }
+
+    // Non-done task → use worktree-backed diff
     if (!worktree) {
       setLoading(false);
       return;
@@ -52,30 +107,37 @@ export function TaskChangesTab({ taskId, worktree, projectId }: TaskChangesTabPr
     try {
       setLoading(true);
       setError(null);
-      const data = await fetchTaskDiff(taskId, undefined, projectId);
-      setDiffData(data);
-      // Auto-expand first file if there are files
-      if (data.files.length > 0) {
-        setExpandedFiles(new Set([data.files[0].path]));
+      const data: TaskDiff = await fetchTaskDiff(taskId, undefined, projectId);
+      const normalized: NormalizedFile[] = data.files.map((f) => ({
+        path: f.path,
+        status: f.status,
+        additions: f.additions,
+        deletions: f.deletions,
+        patch: f.patch,
+      }));
+      setFiles(normalized);
+      setStats(data.stats);
+      if (normalized.length > 0) {
+        setExpandedFiles(new Set([normalized[0].path]));
       }
     } catch (err: any) {
       setError(err.message || "Failed to load diff");
     } finally {
       setLoading(false);
     }
-  }, [taskId, worktree, projectId]);
+  }, [taskId, worktree, projectId, useCommitDiff, commitSha, mergeDetails]);
 
   useEffect(() => {
     loadDiff();
   }, [loadDiff]);
 
-  const toggleFile = (file: string) => {
+  const toggleFile = (filePath: string) => {
     setExpandedFiles((prev) => {
       const next = new Set(prev);
-      if (next.has(file)) {
-        next.delete(file);
+      if (next.has(filePath)) {
+        next.delete(filePath);
       } else {
-        next.add(file);
+        next.add(filePath);
       }
       return next;
     });
@@ -103,7 +165,8 @@ export function TaskChangesTab({ taskId, worktree, projectId }: TaskChangesTabPr
     );
   }
 
-  if (!worktree) {
+  // Non-done task without a worktree → show worktree empty state
+  if (!useCommitDiff && !worktree) {
     return (
       <div className="detail-section">
         <div className="task-changes-state task-changes-state--empty">
@@ -117,14 +180,16 @@ export function TaskChangesTab({ taskId, worktree, projectId }: TaskChangesTabPr
     );
   }
 
-  if (!diffData || diffData.files.length === 0) {
+  if (files.length === 0) {
     return (
       <div className="detail-section">
         <div className="task-changes-state task-changes-state--empty">
           <FileCode size={24} />
           <p>No files modified.</p>
           <span className="task-changes-state-hint">
-            The agent did not modify any files during execution.
+            {useCommitDiff
+              ? "No file changes were recorded in the merge commit."
+              : "The agent did not modify any files during execution."}
           </span>
         </div>
       </div>
@@ -133,10 +198,32 @@ export function TaskChangesTab({ taskId, worktree, projectId }: TaskChangesTabPr
 
   return (
     <div className="detail-section task-changes-tab">
+      {/* Commit metadata for done tasks */}
+      {useCommitDiff && mergeDetails && (
+        <div className="commit-diff-meta">
+          <div className="commit-diff-sha">
+            <GitCommit size={14} />
+            <code>{commitSha!.slice(0, 7)}</code>
+          </div>
+          {mergeDetails.mergeCommitMessage && (
+            <div className="commit-diff-message">{mergeDetails.mergeCommitMessage}</div>
+          )}
+          {mergeDetails.mergedAt && (
+            <div className="commit-diff-timestamp">
+              Merged {new Date(mergeDetails.mergedAt).toLocaleString()}
+            </div>
+          )}
+        </div>
+      )}
+
       <div className="changes-header">
         <h4>
           <FileCode size={16} />
-          Modified Files ({diffData.files.length})
+          Files Changed ({stats.filesChanged})
+          <span className="changes-stat-summary">
+            <span className="diff-add">+{stats.additions}</span>{" "}
+            <span className="diff-del">-{stats.deletions}</span>
+          </span>
         </h4>
         <button
           className="btn btn-sm"
@@ -148,46 +235,43 @@ export function TaskChangesTab({ taskId, worktree, projectId }: TaskChangesTabPr
       </div>
 
       <div className="changes-file-list">
-        {diffData.files.map((fileEntry) => {
-          const { path, status, patch } = fileEntry;
-          const isExpanded = expandedFiles.has(path);
+        {files.map((file) => {
+          const isExpanded = expandedFiles.has(file.path);
 
           return (
             <div
-              key={path}
+              key={file.path}
               className={`changes-file-item ${isExpanded ? "expanded" : ""}`}
             >
               <button
                 className="changes-file-header"
-                onClick={() => toggleFile(path)}
+                onClick={() => toggleFile(file.path)}
               >
                 <span className="changes-file-toggle">
                   {isExpanded ? <ChevronDown size={14} /> : <ChevronRight size={14} />}
                 </span>
                 <span
                   className="changes-file-status"
-                  style={{ color: getStatusColor(status) }}
-                  title={status}
+                  style={{ color: getStatusColor(file.status) }}
+                  title={file.status}
                 >
-                  {status === "added" && "A"}
-                  {status === "modified" && "M"}
-                  {status === "deleted" && "D"}
+                  {getStatusLabel(file.status)}
                 </span>
-                <span className="changes-file-path" title={path}>
-                  {path}
+                <span className="changes-file-path" title={file.path}>
+                  {file.path}
                 </span>
                 <span
                   className="changes-file-stat"
-                  title={`+${fileEntry.additions} -${fileEntry.deletions}`}
+                  title={`+${file.additions} -${file.deletions}`}
                 >
-                  +{fileEntry.additions} -{fileEntry.deletions}
+                  +{file.additions} -{file.deletions}
                 </span>
               </button>
 
-              {isExpanded && patch && (
+              {isExpanded && file.patch && (
                 <div className="changes-file-content">
                   <pre className="changes-diff-patch">
-                    <code>{highlightDiff(patch)}</code>
+                    <code>{highlightDiff(file.patch)}</code>
                   </pre>
                 </div>
               )}

packages/dashboard/app/components/TaskDetailModal.tsx

@@ -789,7 +789,7 @@ export function TaskDetailModal({
               />
             </div>
           ) : activeTab === "changes" ? (
-            <TaskChangesTab taskId={task.id} worktree={task.worktree} projectId={projectId} />
+            <TaskChangesTab taskId={task.id} worktree={task.worktree} projectId={projectId} column={task.column} mergeDetails={task.mergeDetails} />
           ) : activeTab === "commits" ? (
             <CommitDiffTab commitSha={task.mergeDetails?.commitSha ?? ""} mergeDetails={task.mergeDetails} />
           ) : activeTab === "comments" ? (

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

@@ -0,0 +1,398 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { render, screen, fireEvent, waitFor } from "@testing-library/react";
+import { TaskChangesTab } from "../TaskChangesTab";
+import type { MergeDetails, Column } from "@fusion/core";
+
+const mockFetchTaskDiff = vi.fn();
+const mockFetchCommitDiff = vi.fn();
+
+vi.mock("../../api", () => ({
+  fetchTaskDiff: (...args: any[]) => mockFetchTaskDiff(...args),
+  fetchCommitDiff: (...args: any[]) => mockFetchCommitDiff(...args),
+}));
+
+vi.mock("lucide-react", () => ({
+  FileCode: () => null,
+  ChevronDown: ({ size }: any) => <span data-testid="chevron-down" />,
+  ChevronRight: ({ size }: any) => <span data-testid="chevron-right" />,
+  AlertCircle: () => null,
+  GitCommit: () => null,
+}));
+
+vi.mock("../../utils/highlightDiff", () => ({
+  highlightDiff: (diff: string) => diff,
+}));
+
+vi.mock("../CommitDiffTab", () => ({
+  parsePatch: (rawPatch: string) => {
+    const files: Array<{
+      path: string;
+      status: "added" | "modified" | "deleted" | "unknown";
+      additions: number;
+      deletions: number;
+      patch: string;
+    }> = [];
+    const parts = rawPatch.split(/(?=^diff --git )/m);
+    for (const part of parts) {
+      const trimmed = part.trim();
+      if (!trimmed.startsWith("diff --git ")) continue;
+      const headerMatch = trimmed.match(/^diff --git a\/(.+?) b\/(.+)/m);
+      const path = headerMatch ? headerMatch[2] : "unknown";
+      let status: "added" | "modified" | "deleted" | "unknown" = "modified";
+      if (trimmed.includes("new file mode")) status = "added";
+      else if (trimmed.includes("deleted file mode")) status = "deleted";
+      let additions = 0;
+      let deletions = 0;
+      for (const line of trimmed.split("\n")) {
+        if (line.startsWith("+") && !line.startsWith("+++")) additions++;
+        else if (line.startsWith("-") && !line.startsWith("---")) deletions++;
+      }
+      files.push({ path, status, additions, deletions, patch: trimmed });
+    }
+    return files;
+  },
+}));
+
+const SAMPLE_PATCH = `diff --git a/src/app.ts b/src/app.ts
+index abc1234..def5678 100644
+--- a/src/app.ts
++++ b/src/app.ts
+@@ -1,3 +1,4 @@
+ import express from "express";
++import cors from "cors";
+ const app = express();
+ app.listen(3000);
+diff --git a/src/new-file.ts b/src/new-file.ts
+new file mode 100644
+index 0000000..abc1234
+--- /dev/null
++++ b/src/new-file.ts
+@@ -0,0 +1,2 @@
++export function hello() {}
++export function world() {}`;
+
+const MERGE_DETAILS: MergeDetails = {
+  commitSha: "abc1234567890def",
+  filesChanged: 2,
+  insertions: 3,
+  deletions: 0,
+  mergeCommitMessage: "Merge branch 'fusion/fn-001' into main",
+  mergedAt: "2026-01-15T10:30:00Z",
+};
+
+beforeEach(() => {
+  mockFetchTaskDiff.mockReset();
+  mockFetchCommitDiff.mockReset();
+});
+
+describe("TaskChangesTab — worktree-backed (non-done tasks)", () => {
+  it("shows 'No worktree available' when no worktree and not done", () => {
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="in-progress"
+      />,
+    );
+    expect(screen.getByText("No worktree available for this task.")).toBeTruthy();
+    expect(mockFetchTaskDiff).not.toHaveBeenCalled();
+    expect(mockFetchCommitDiff).not.toHaveBeenCalled();
+  });
+
+  it("loads diff from fetchTaskDiff for in-progress task with worktree", async () => {
+    mockFetchTaskDiff.mockResolvedValue({
+      files: [
+        { path: "src/app.ts", status: "modified", additions: 1, deletions: 0, patch: "@@ -1 +1,2 @@" },
+      ],
+      stats: { filesChanged: 1, additions: 1, deletions: 0 },
+    });
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree="/path/to/worktree"
+        column="in-progress"
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("src/app.ts")).toBeTruthy();
+    });
+    expect(mockFetchTaskDiff).toHaveBeenCalledWith("FN-001", undefined, undefined);
+    expect(mockFetchCommitDiff).not.toHaveBeenCalled();
+  });
+
+  it("loads diff from fetchTaskDiff for in-review task with worktree", async () => {
+    mockFetchTaskDiff.mockResolvedValue({
+      files: [
+        { path: "src/app.ts", status: "modified", additions: 1, deletions: 0, patch: "@@ -1 +1,2 @@" },
+      ],
+      stats: { filesChanged: 1, additions: 1, deletions: 0 },
+    });
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree="/path/to/worktree"
+        column="in-review"
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("src/app.ts")).toBeTruthy();
+    });
+    expect(mockFetchTaskDiff).toHaveBeenCalled();
+    expect(mockFetchCommitDiff).not.toHaveBeenCalled();
+  });
+
+  it("shows 'No files modified' when worktree diff returns empty", async () => {
+    mockFetchTaskDiff.mockResolvedValue({
+      files: [],
+      stats: { filesChanged: 0, additions: 0, deletions: 0 },
+    });
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree="/path/to/worktree"
+        column="in-progress"
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("No files modified.")).toBeTruthy();
+    });
+  });
+
+  it("shows error state when fetchTaskDiff fails", async () => {
+    mockFetchTaskDiff.mockRejectedValue(new Error("Network error"));
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree="/path/to/worktree"
+        column="in-progress"
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText(/Error loading changes: Network error/)).toBeTruthy();
+    });
+  });
+
+  it("shows loading state initially", () => {
+    mockFetchTaskDiff.mockReturnValue(new Promise(() => {})); // never resolves
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree="/path/to/worktree"
+        column="in-progress"
+      />,
+    );
+    expect(screen.getByText("Loading changes...")).toBeTruthy();
+  });
+});
+
+describe("TaskChangesTab — commit-backed (done tasks)", () => {
+  it("loads diff from fetchCommitDiff for done task with commitSha", async () => {
+    mockFetchCommitDiff.mockResolvedValue({ stat: "", patch: SAMPLE_PATCH });
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("src/app.ts")).toBeTruthy();
+    });
+    expect(mockFetchCommitDiff).toHaveBeenCalledWith("abc1234567890def");
+    expect(mockFetchTaskDiff).not.toHaveBeenCalled();
+  });
+
+  it("shows commit metadata for done task", async () => {
+    mockFetchCommitDiff.mockResolvedValue({ stat: "", patch: SAMPLE_PATCH });
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("abc1234")).toBeTruthy(); // short SHA
+    });
+    expect(screen.getByText("Merge branch 'fusion/fn-001' into main")).toBeTruthy();
+    expect(screen.getByText(/Merged .+/)).toBeTruthy();
+  });
+
+  it("uses mergeDetails stats for summary", async () => {
+    mockFetchCommitDiff.mockResolvedValue({ stat: "", patch: SAMPLE_PATCH });
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("Files Changed (2)")).toBeTruthy();
+    });
+    expect(screen.getByText("+3")).toBeTruthy();
+    expect(screen.getByText("-0")).toBeTruthy();
+  });
+
+  it("toggling file expansion shows/hides diff content", async () => {
+    mockFetchCommitDiff.mockResolvedValue({ stat: "", patch: SAMPLE_PATCH });
+    const { container } = render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("src/app.ts")).toBeTruthy();
+    });
+
+    // First file should be auto-expanded
+    let diffBlocks = container.querySelectorAll(".changes-file-content");
+    expect(diffBlocks.length).toBe(1);
+
+    // Click to collapse
+    fireEvent.click(screen.getByText("src/app.ts"));
+    diffBlocks = container.querySelectorAll(".changes-file-content");
+    expect(diffBlocks.length).toBe(0);
+
+    // Click on second file to expand
+    fireEvent.click(screen.getByText("src/new-file.ts"));
+    diffBlocks = container.querySelectorAll(".changes-file-content");
+    expect(diffBlocks.length).toBe(1);
+  });
+
+  it("shows 'No files modified' with commit-specific hint when patch is empty", async () => {
+    mockFetchCommitDiff.mockResolvedValue({ stat: "", patch: "" });
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("No files modified.")).toBeTruthy();
+    });
+    expect(screen.getByText("No file changes were recorded in the merge commit.")).toBeTruthy();
+  });
+
+  it("shows error state when fetchCommitDiff fails", async () => {
+    mockFetchCommitDiff.mockRejectedValue(new Error("Git error"));
+
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText(/Error loading changes: Git error/)).toBeTruthy();
+    });
+  });
+
+  it("renders commit SHA metadata even when only commitSha is set", async () => {
+    mockFetchCommitDiff.mockResolvedValue({ stat: "", patch: SAMPLE_PATCH });
+
+    const { container } = render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={{ commitSha: "abc1234567890def" }}
+      />,
+    );
+
+    await waitFor(() => {
+      expect(screen.getByText("src/app.ts")).toBeTruthy();
+    });
+
+    // SHA metadata should render since commitSha is present
+    expect(container.querySelector(".commit-diff-meta")).toBeTruthy();
+    expect(screen.getByText("abc1234")).toBeTruthy(); // short SHA
+    // But message and timestamp should NOT be present since they're not set
+    expect(container.querySelector(".commit-diff-message")).toBeNull();
+    expect(container.querySelector(".commit-diff-timestamp")).toBeNull();
+  });
+});
+
+describe("TaskChangesTab — regression: non-done tasks still use worktree path", () => {
+  it("in-progress without worktree shows worktree empty state, not commit path", () => {
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="in-progress"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+    expect(screen.getByText("No worktree available for this task.")).toBeTruthy();
+    expect(mockFetchCommitDiff).not.toHaveBeenCalled();
+  });
+
+  it("in-review without worktree shows worktree empty state", () => {
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="in-review"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+    expect(screen.getByText("No worktree available for this task.")).toBeTruthy();
+    expect(mockFetchCommitDiff).not.toHaveBeenCalled();
+  });
+
+  it("todo task never loads diff even with mergeDetails", () => {
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="todo"
+        mergeDetails={MERGE_DETAILS}
+      />,
+    );
+    expect(screen.getByText("No worktree available for this task.")).toBeTruthy();
+    expect(mockFetchTaskDiff).not.toHaveBeenCalled();
+    expect(mockFetchCommitDiff).not.toHaveBeenCalled();
+  });
+
+  it("done task without commitSha falls through to worktree path", () => {
+    render(
+      <TaskChangesTab
+        taskId="FN-001"
+        worktree={undefined}
+        column="done"
+        mergeDetails={{}} // no commitSha
+      />,
+    );
+    // Falls through to the !worktree check for non-commit-diff path
+    expect(screen.getByText("No worktree available for this task.")).toBeTruthy();
+    expect(mockFetchCommitDiff).not.toHaveBeenCalled();
+  });
+});