fix(merger,dashboard): handle empty-squash commitSha to keep card and Changes tab in sync

When a branch contained commits already on main (duplicate cherry-picks),
the merger's local squash collapsed to an empty commit. The merger then
recorded that empty commit's SHA on mergeDetails.commitSha. The actual
content landed later on main as a different SHA via PR merge, but the
task kept pointing at the orphaned empty commit.

Symptom: TaskCard showed "N files changed" (falling back to
task.modifiedFiles), but the Changes tab in the modal showed nothing
because the API hit `git diff sha^..sha` on the empty commit and
returned no files.

Two fixes:

1. merger.ts: detect empty squash commits and skip storing commitSha,
   logging clearly. recoverInterruptedMergingTasks → findLandedTaskCommit
   already exists to backfill the right SHA when the real commit lands;
   a missing commitSha is a known fallback path the UI already handles.

2. TaskChangesTab.tsx: when the API returns no files for a done task,
   fall back to task.modifiedFiles (paths only, no patches) with a clear
   note. Mirrors the existing 3-tier fallback in TaskCard.tsx:1090-1124
   so card and modal always agree.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/TaskChangesTab.tsx

@@ -13,6 +13,15 @@ interface TaskChangesTabProps {
   projectId?: string;
   column?: Column;
   mergeDetails?: MergeDetails;
+  /**
+   * Files modified by the task during execution, captured from the worktree.
+   * Used as a last-resort fallback when the recorded `mergeDetails.commitSha`
+   * resolves to an empty git commit (which can happen when the merger stores
+   * a per-branch SHA that gets collapsed into a different squash on main).
+   * Without this, the tab would show "no changes" while the card shows N —
+   * matches TaskCard.tsx:1090-1124's fallback ladder.
+   */
+  modifiedFiles?: string[];
 }
 
 function getStatusLabel(status: "added" | "modified" | "deleted" | "unknown"): string {
@@ -51,7 +60,7 @@ interface NormalizedFile {
  * changes. This prevents inflated file counts that don't match the card-level
  * display.
  */
-export function TaskChangesTab({ taskId, worktree, projectId, column, mergeDetails }: TaskChangesTabProps) {
+export function TaskChangesTab({ taskId, worktree, projectId, column, mergeDetails, modifiedFiles }: 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);
@@ -192,6 +201,57 @@ export function TaskChangesTab({ taskId, worktree, projectId, column, mergeDetai
   }
 
   if (files.length === 0) {
+    // Done task with a commit SHA but the diff came back empty — almost
+    // always means the recorded SHA points to an empty commit (merger
+    // stored a per-branch SHA that became no-op after the squash collapsed
+    // onto main). Fall back to the executor-captured modifiedFiles so the
+    // tab agrees with the card. Patches are unavailable in this path.
+    if (isDone && modifiedFiles && modifiedFiles.length > 0) {
+      return (
+        <div className="detail-section task-changes-tab">
+          {isDone && mergeDetails && (
+            <div className="commit-diff-meta">
+              {mergeDetails.commitSha && (
+                <div className="commit-diff-sha">
+                  <GitCommit size={14} />
+                  <code>{mergeDetails.commitSha.slice(0, 7)}</code>
+                </div>
+              )}
+              {mergeDetails.mergedAt && (
+                <div className="commit-diff-timestamp">
+                  Merged {new Date(mergeDetails.mergedAt).toLocaleString()}
+                </div>
+              )}
+            </div>
+          )}
+          <div className="task-changes-state task-changes-state--empty">
+            <FileCode size={24} />
+            <p>{modifiedFiles.length} file{modifiedFiles.length === 1 ? "" : "s"} modified during execution.</p>
+            <span className="task-changes-state-hint">
+              The recorded merge commit has no diff (likely collapsed into a squash on main). Showing file paths only — patches unavailable.
+            </span>
+          </div>
+          <div className="changes-file-list task-changes-file-list--compact">
+            {modifiedFiles.map((path) => (
+              <div key={path} className="changes-file-item">
+                <div className="changes-file-header">
+                  <span
+                    className="changes-file-status changes-file-status--unknown"
+                    title="status unknown"
+                  >
+                    {getStatusLabel("unknown")}
+                  </span>
+                  <span className="changes-file-path" title={path}>
+                    {truncateMiddle(path)}
+                  </span>
+                </div>
+              </div>
+            ))}
+          </div>
+        </div>
+      );
+    }
+
     return (
       <div className="detail-section">
         <div className="task-changes-state task-changes-state--empty">

packages/dashboard/app/components/TaskDetailModal.tsx

@@ -1355,7 +1355,7 @@ export function TaskDetailModal({
               )}
             </div>
           ) : activeTab === "changes" ? (
-            <TaskChangesTab taskId={task.id} worktree={task.worktree} projectId={projectId} column={task.column} mergeDetails={task.mergeDetails} />
+            <TaskChangesTab taskId={task.id} worktree={task.worktree} projectId={projectId} column={task.column} mergeDetails={task.mergeDetails} modifiedFiles={task.modifiedFiles} />
           ) : activeTab === "comments" ? (
             <TaskComments task={task} addToast={addToast} projectId={projectId} onTaskUpdated={onTaskUpdated} />
           ) : activeTab === "documents" ? (

packages/engine/src/merger.ts

@@ -2292,8 +2292,25 @@ export async function aiMergeTask(
       deletions = deletionsMatch ? Number.parseInt(deletionsMatch[1], 10) : 0;
     } catch { /* non-fatal */ }
 
+    // Guard: if the squash collapsed to an empty commit, recording its SHA
+    // misleads every consumer (TaskChangesTab shows "no changes" even though
+    // modifiedFiles is non-empty). Real cause: the branch contained commits
+    // already on main (duplicate cherry-picks), and conflict resolution
+    // dropped them. The actual landing typically happens later via PR merge
+    // on a different SHA — let recoverInterruptedMergingTasks /
+    // findLandedTaskCommit populate the right SHA when that lands. Until
+    // then, store mergeDetails without commitSha so the UI falls back to
+    // task.modifiedFiles instead of a broken diff.
+    const isEmptyCommit = filesChanged === 0;
+    const recordedSha = isEmptyCommit ? undefined : commitSha;
+    if (isEmptyCommit) {
+      mergerLog.warn(
+        `${taskId}: local squash produced an empty commit (${commitSha?.slice(0, 8)}) — branch likely contained dupes of main. Skipping commitSha; recovery will backfill when real commit lands.`,
+      );
+    }
+
     const mergeDetails: MergeDetails = {
-      commitSha,
+      commitSha: recordedSha,
       filesChanged,
       insertions,
       deletions,
@@ -2307,7 +2324,7 @@ export async function aiMergeTask(
     };
 
     await store.updateTask(taskId, { mergeDetails });
-    mergerLog.log(`${taskId}: merge details stored (commitSha: ${commitSha?.slice(0, 8)})`);
+    mergerLog.log(`${taskId}: merge details stored (commitSha: ${recordedSha?.slice(0, 8) ?? "<deferred>"})`);
   } catch (err: any) {
     mergerLog.warn(`${taskId}: failed to collect/store merge details: ${err.message}`);
   }