fix(downloads): stop active tasks from starving terminal rows out of the list

Root cause of "completed/failed/unverified don't show during a running
batch, only after it ends" (F5 didn't help): build_unified_downloads_response
sorted live tasks active-first (downloading/searching/queued = priority 0-3,
completed/failed = 4-7) then truncated the whole array at items[:limit] (300).
During a busy batch the active+queued tasks filled the limit and pushed every
terminal task off the end, so /api/downloads/all never returned them — the
Completed/Failed/Unverified tabs filter client-side and had nothing to show.

Fix: `limit` now bounds only the persistent-history tail. Live in-memory
tasks are always returned in full — they're already bounded by the 5-min
cleanup automation, and array order is presentation-only since the page
filters per tab client-side.

Verified with a repro (320 queued + 1 completed + 1 failed → terminal rows
were absent at limit=300; now present).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/downloads/status.py

@@ -839,7 +839,14 @@ def build_unified_downloads_response(limit: int, deps: StatusDeps) -> dict:
             live_identities.add(identity)
             appended_history += 1
 
-    # Sort: active first (by priority), then by timestamp desc within each group
+    # Sort: active first (by priority), then by timestamp desc within each group.
+    # NOTE: the array order is presentation-only — the Downloads page filters
+    # client-side per tab. What matters is that EVERY live task is present: an
+    # earlier `items[:limit]` truncation (active-first) starved completed/failed/
+    # unverified rows off the end during a busy batch, so those tabs stayed empty
+    # until the batch drained. `limit` now bounds only the persistent-history
+    # tail (handled above); live in-memory tasks are always returned in full
+    # (they're already bounded by the 5-min cleanup automation).
     items.sort(key=lambda x: (x['priority'], -x['timestamp']))
 
     # Build batch summaries for the batch context panel
@@ -867,7 +874,7 @@ def build_unified_downloads_response(limit: int, deps: StatusDeps) -> dict:
 
     return {
         'success': True,
-        'downloads': items[:limit],
+        'downloads': items,
         'total': len(items),
         'batches': batch_summaries,
         'timestamp': time.time(),

tests/downloads/test_downloads_status.py

@@ -667,15 +667,44 @@ def test_unified_response_includes_batch_summaries():
     assert bs['queued'] == 1
 
 
-def test_unified_response_respects_limit():
+def test_unified_response_returns_all_live_tasks_even_past_limit():
+    """`limit` bounds the persistent-history tail, NOT live in-memory tasks.
+
+    Live tasks are already bounded by the 5-min cleanup, and the Downloads page
+    filters them client-side per tab — truncating them (active-first) starved
+    completed/failed/unverified rows out of the response during a busy batch so
+    they never showed until the batch drained.
+    """
     deps, _ = _build_deps()
     for i in range(20):
         download_tasks[f't{i}'] = {
             'track_index': i, 'status': 'completed', 'track_info': {},
         }
     out = st.build_unified_downloads_response(5, deps)
-    assert len(out['downloads']) == 5
-    assert out['total'] == 20  # total still reflects all
+    assert len(out['downloads']) == 20  # all live tasks returned
+    assert out['total'] == 20
+
+
+def test_unified_response_does_not_truncate_terminal_tasks_behind_active():
+    """A busy batch (many queued/active tasks) must not push completed/failed
+    rows off the end of the response — they're what the Completed/Failed tabs
+    show during the run."""
+    deps, _ = _build_deps()
+    for i in range(120):
+        download_tasks[f'q{i}'] = {
+            'track_index': i, 'status': 'queued',
+            'track_info': {'name': f'Q{i}'}, 'status_change_time': i,
+        }
+    download_tasks['done'] = {
+        'status': 'completed', 'track_info': {'name': 'DONE'}, 'status_change_time': 999,
+    }
+    download_tasks['fail'] = {
+        'status': 'failed', 'track_info': {'name': 'FAIL'}, 'status_change_time': 999,
+    }
+    out = st.build_unified_downloads_response(100, deps)
+    titles = {d['title'] for d in out['downloads']}
+    assert 'DONE' in titles
+    assert 'FAIL' in titles
 
 
 def test_unified_response_includes_persistent_download_history():