phase: 166_hide_empty_answer_bubble

**Phase 166 — final verification pass: green.** All 3 task files already in `complete/`, so this was verification + defect/doc fixing.

- **Verified:** `.msg .bubble:empty { display: none; }` exists once in the Messages block with provenance; `frontend/` diff is styles.css only (**no app.js change**); `tests/unit/test_no_empty_answer_bubble.py` 26 pins pass; phase-158/161 anchor pins (thinking_timeline + tool_states + edit_message) **75 passed**.
- **Phase E2E in isolation:** `uv run pytest tests/e2e/test_no_empty_box.py --no-cov` → **5 passed**. Mutation check (rule removed) → 2 failures, so the suite genuinely pins the defect.
- **Regression set, isolated:** thinking_timeline 6 · turn_progress_loader 3 · chat_history 5 · share_chat 4 · save_share_ux 5 · failed_turn_retry 4 · chat_answer_clusters 4 · chat_error_honesty 4 → all pass.
- **Full gates:** `uv run pytest --cov=app --cov-report=term-missing` → **3880 passed, app/ 99%** (gate >90%); `uv run ruff check .` → clean; `uv run pyright` → **0 errors**.
- **Whole E2E tree swept file-by-file (150 files, own pytest process each).** 4 suites were broken by *earlier* phases; fixed and re-run green:
  - `test_kb_tree`, `test_sync_summary_visibility`, `test_folder_summary_voice` — `td:nth-child(4) span` became a strict-mode violation when phase 156 wrapped the Edit label in `.cell-clip` → scoped to `span.kb-desc-text`.
  - `test_search_tool_scope` — pinned the pre-`690fd98` per-run `<summary>`/`open` shape → re-pinned to the ONE `details.reasoning` container + `.tool-calls-summary`.
  - `test_header_consistency` — `.doc-meta` pixel bound (`<34px`) was broken by phase 153's 44px touch-target floor at ≤640px → replaced with a shape-first single-line check that still fails a real wrap.
- **Completion criteria:** mid-turn box hidden ✅ · bubble appears at first delta, phase-130 rows below unchanged ✅ · restored/shared empty answer paints no box ✅ · no app.js change + anchors + full gates ✅ · commit/`complete/` move = harness's job (I ran no git index/history commands).
- **Docs:** README — added the phase-166 "no empty answer box mid-turn" note; fixed a **stale** Thinking lead ("collapsible Thinking blocks", pre-dated the 2026-10-06 one-disclosure refinement); QA section now states E2E isolation is mandatory (shared Postgres + fixed ports 8123/8901) with the per-file sweep loop. AGENTS.md — rule 5 UI checklist now names the phase-166 suppression rule.
- **Notable:** running all of `tests/e2e/` in one pytest session is not supported (fixture/port interleaving causes ~126 spurious failures) — documented in README. Restored the tracked `.agents/screenshots/doc_modal_scrollbar_1280.png` that `test_document_viewer.py` rewrites on every run.
- **Next pending phase:** `167_api_payload_diet`.

tests/e2e/test_kb_tree.py

@@ -545,10 +545,10 @@ def test_top_level_lists_sources_with_descriptions(
     rows = page.locator("#folders-tbody tr")
     expect(rows.nth(0).locator("a.folder-link")).to_have_text(ALPHA)
     expect(rows.nth(0).locator("td:nth-child(2)")).to_have_text(str(ALPHA_COUNT))
-    expect(rows.nth(0).locator("td:nth-child(4) span")).to_have_text(ALPHA_ROOT_SUM)
+    expect(rows.nth(0).locator("td:nth-child(4) span.kb-desc-text")).to_have_text(ALPHA_ROOT_SUM)
     expect(rows.nth(1).locator("a.folder-link")).to_have_text(BETA)
     expect(rows.nth(1).locator("td:nth-child(2)")).to_have_text(str(BETA_COUNT))
-    expect(rows.nth(1).locator("td:nth-child(4) span")).to_have_text(BETA_ROOT_SUM)
+    expect(rows.nth(1).locator("td:nth-child(4) span.kb-desc-text")).to_have_text(BETA_ROOT_SUM)
     # The edit affordance is ALWAYS present (a description can be
     # CREATED where none is stored) — the row surface, one per row.
     expect(page.locator("#folders-tbody .kb-summary-edit")).to_have_count(2)
@@ -607,10 +607,10 @@ def test_drill_into_source(
     expect(rows).to_have_count(2)
     expect(rows.nth(0).locator("a.folder-link")).to_have_text("one")
     expect(rows.nth(0).locator("td:nth-child(2)")).to_have_text("2")
-    expect(rows.nth(0).locator("td:nth-child(4) span")).to_have_text(ONE_SUM)
+    expect(rows.nth(0).locator("td:nth-child(4) span.kb-desc-text")).to_have_text(ONE_SUM)
     expect(rows.nth(1).locator("a.folder-link")).to_have_text("two")
     expect(rows.nth(1).locator("td:nth-child(2)")).to_have_text("2")
-    expect(rows.nth(1).locator("td:nth-child(4) span")).to_have_text(TWO_SUM)
+    expect(rows.nth(1).locator("td:nth-child(4) span.kb-desc-text")).to_have_text(TWO_SUM)
 
     # The level's direct files: root-note.md — the 6-column contract
     # (makeRow): Source | Path | Title | Chunks | Created | Indexed
@@ -718,7 +718,7 @@ def test_edit_folder_description(
     # (the textContent re-render — the canned text is gone); the
     # always-present Edit button is back.
     expect(status).to_have_text("Description updated.")
-    expect(row.locator("td:nth-child(4) span")).to_have_text(NEW_ALPHA_ONE)
+    expect(row.locator("td:nth-child(4) span.kb-desc-text")).to_have_text(NEW_ALPHA_ONE)
     expect(row.locator(".kb-summary-edit")).to_be_visible()
 
     # The server row: the new text AND the manually_edited flag (the
@@ -761,7 +761,7 @@ def test_clear_folder_description(
     # The text is gone: the cell empties (the always-present Edit
     # button stays — a description can be re-created from the empty
     # cell).
-    expect(row.locator("td:nth-child(4) span")).to_have_text("")
+    expect(row.locator("td:nth-child(4) span.kb-desc-text")).to_have_text("")
     expect(row.locator(".kb-summary-edit")).to_be_visible()
 
     # The SQL assert: no row for the folder (the next KB-changing
@@ -794,7 +794,7 @@ def test_manual_description_survives_a_changed_sync(
     rows = page.locator("#folders-tbody tr")
     expect(rows).to_have_count(1)
     expect(rows.nth(0).locator("a.folder-link")).to_have_text("gamma")
-    expect(rows.nth(0).locator("td:nth-child(4) span")).to_have_text(GAMMA_SUM)
+    expect(rows.nth(0).locator("td:nth-child(4) span.kb-desc-text")).to_have_text(GAMMA_SUM)
 
     # The LEVEL-BLOCK Edit (the level surface — test 4 pins the row
     # surface): the static #kb-level-edit button, the source root
@@ -836,7 +836,7 @@ def test_manual_description_survives_a_changed_sync(
     expect(
         page.locator("#folders-tbody tr")
         .nth(0)
-        .locator("td:nth-child(4) span")
+        .locator("td:nth-child(4) span.kb-desc-text")
     ).to_have_text(GAMMA_SUM)
 
     # The new document landed in the tree: back to the top (the
@@ -847,7 +847,7 @@ def test_manual_description_survives_a_changed_sync(
     one_row = page.locator("#folders-tbody tr").nth(0)
     expect(one_row.locator("a.folder-link")).to_have_text("one")
     expect(one_row.locator("td:nth-child(2)")).to_have_text("3")
-    expect(one_row.locator("td:nth-child(4) span")).to_have_text(ONE_SUM)
+    expect(one_row.locator("td:nth-child(4) span.kb-desc-text")).to_have_text(ONE_SUM)
 
     # The SQL asserts: the manual row survived (text + flag), the
     # untouched rows were regenerated (the canned text, the flag back