fix(ui): thinking window no longer scrolls — live 320px view pinned to the stream tail

.agent/phases/complete/21_thinking_no_scroll/02_e2e_story_suite_commit.md

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+# Task 02 — E2E story suite, story file, validation, commit
+
+**Phase:** `21_thinking_no_scroll` · **Source:** `TODO.md` L4
+
+## Objective
+The story gate: `tests/e2e/test_thinking_no_scroll.py` proves the window
+can't be user-scrolled but always tracks the live tail, plus regressions,
+story file, final validation, and the single atomic commit.
+
+## Work
+1. `tests/e2e/test_thinking_no_scroll.py` (new — reuse
+   `test_thinking_display.py`'s mock-LLM streaming scaffolding; the mock
+   must stream a **long** thinking body, in many chunks, so
+   `.thinking-text` overflow exceeds its 320px box). Tests:
+   1. `test_thinking_window_not_user_scrollable` — open the block, wait
+      until `scrollHeight > clientHeight`; focus `.thinking-text`
+      (`el.focus()`), dispatch mouse wheel over it
+      (`page.mouse.wheel(0, -200)` after moving the mouse over the
+      element) and press `Home`/`ArrowUp`: `scrollTop` must not decrease
+      (assert `scrollTop` unchanged within 1px between actions).
+   2. `test_thinking_window_tracks_live_tail` — while chunks stream,
+      after the 2nd-to-last and last chunk:
+      `scrollTop === scrollHeight` (within 1px) — the visible window is
+      the live tail; the **last** chunk's text is within the visible
+      rectangle (its offsetTop + scrollTop geometry check, or
+      `elementFromPoint` at the box's bottom).
+   3. `test_thinking_window_css_contract` — computed style of
+      `.thinking-text`: `overflow-y === "hidden"`,
+      `max-height === "320px"`.
+   4. `test_answer_bubble_still_scrollable` (regression, phase 11) — a
+      long answer (use the long-answer mock from
+      `test_long_answers.py`): the answer bubble is still
+      user-scrollable (scrollTop moves on wheel) and
+      `overflow-y` is not `hidden` there.
+   5. `test_restored_collapsed_thinking_unaffected` (regression,
+      phase 17) — a turn with stored `thinking`, reload: the collapsed
+      Thinking block renders with its text (existing pin from
+      `test_thinking_display.py` — replicate, don't duplicate the file).
+2. `.agent/user_stories/thinking-no-scroll.md` (new) — story file per
+   the repo format: goal, the bug report verbatim from `TODO.md` L4, the
+   owner-confirmed A2 decisions from `00_phase.md`, E2E mapping table.
+3. Run the suite **in isolation** (prereq `podman compose up -d db`):
+   `uv run pytest tests/e2e/test_thinking_no_scroll.py -v --no-cov`.
+4. Regressions, in isolation, one command each:
+   - `uv run pytest tests/e2e/test_thinking_display.py -v --no-cov`
+   - `uv run pytest tests/e2e/test_long_answers.py -v --no-cov`
+5. Final validation: `uv run pytest` green; `uv run pytest --cov=app
+   --cov-report=term-missing` ≥ today's number (>90% gate);
+   `uv run ruff check . && uv run pyright` clean.
+6. **UI Structure Check** (AGENTS.md rule 5): no new surface; the block
+   keeps its summary chevron, focus-visible ring, aria-live/label
+   contract, and the reduced-motion stillness (styles.css ~line 686).
+7. Write the phase report (`.agent/reports/21_thinking_no_scroll/`).
+8. Commit (one atomic commit) and move the phase:
+   ```bash
+   git add -A .agent/ frontend/ tests/
+   git commit --no-gpg-sign -m "fix(ui): thinking window no longer scrolls — live 320px view pinned to the stream tail"
+   mv .agent/phases/todo/21_thinking_no_scroll .agent/phases/complete/
+   ```
+
+## Testing & Quality
+- Story suite green **in isolation**; both regression suites green in
+  isolation; full unit+integration suite green; `app/` coverage at or
+  above today's number (>90%); ruff + pyright clean.
+
+## Completion Criteria
+- [ ] `test_thinking_no_scroll.py` 5/5 in isolation.
+- [ ] Regressions (thinking display, long answers) green in isolation.
+- [ ] Story file + phase report exist.
+- [ ] One `--no-gpg-sign` commit; phase directory in `complete/`.