# Phase 21 — Thinking Window: No Scroll Back, Just Live

**Source:** `TODO.md` L4 — *"Disable scroll in the thinking window.
Users don't need to scroll back through thinking, just see it live."*
**Story:** `.agents/user_stories/thinking-no-scroll.md` (created by task 02)
**Context:** `frontend/assets/styles.css` —
`details.thinking .thinking-text` (today: `max-height: 320px;
overflow-y: auto;`, i.e. a user-scrollable 320px window, ~line 444);
`frontend/assets/app.js` — the phase-17 thinking block (`ensureThinkingBlock`,
the streaming `thinking` branch that appends chunks and already pins the
stream to the bottom: `textEl.scrollTop = textEl.scrollHeight`, ~line 886),
and the phase-14 restore path (stored `thinking` re-renders a
**collapsed** block).

## Objective
The live Thinking block is a scratchpad, not a transcript. The user must
not be able to scroll back through it — the 320px window always shows the
**live tail** of the reasoning stream (the existing per-chunk
bottom-pinning stays). Wheel, drag, and keyboard scrolling on
`.thinking-text` stop working; the stream itself keeps pinning to the
bottom as chunks arrive.

## Owner-confirmed (2026-08-24, roadmap A2)
1. **Keep the 320px clip** — "just see it live" means the window stays a
   fixed 320px viewport showing the newest lines; no auto-height growth,
   no "↓ more" affordance.
2. **The answer bubble is untouched** — final answers keep their existing
   scroll behavior (phase 11 long answers).
3. **Restored (collapsed) Thinking blocks are untouched** — the phase-14
   restore renders them collapsed, where overflow is moot.

## Design
- **CSS (the whole functional change):**
  `details.thinking .thinking-text` — `overflow-y: auto` →
  `overflow-y: hidden`; keep `max-height: 320px`.
  `overflow: hidden` still permits **programmatic** scrolling
  (`scrollTop`), so the phase-17 pin
  (`textEl.scrollTop = textEl.scrollHeight` on every thinking chunk)
  keeps the window glued to the live tail — no JS change needed.
  Add a CSS comment: *no user scroll back (owner choice 2026-08-24):
  the window is a live tail only — the JS bottom-pin is the sole
  scroller*.
- **No JS change** — the pin already exists; nothing else touches
  `.thinking-text` scroll.
- **Non-goals:** no change to the answer bubble, the collapsed restore
  state, the summary/chevron, or the auto-collapse on first delta
  (phase 17).

## Dependencies
- `17_thinking_display` (complete) — the block, the pin, the restore.
- `18_follow_bottom_scroll` (complete) — no overlap (chat-page scroll
  gate only; the thinking window is a separate inner element).
- `11_long_answers` (complete) — the untouched answer-bubble behavior.

## Tasks
1. `01_disable_thinking_scroll.md` — the CSS change + source-level unit
   pins.
2. `02_e2e_story_suite_commit.md` — `tests/e2e/test_thinking_no_scroll.py`
   (the story gate, isolated), regression suites, story file, final
   validation, the single atomic commit, phase move to `complete/`.

## Locked decisions
- **A11 untouched** — no CDN, pure CSS. **A16 honored** — one new story
  E2E suite + adapted regressions. No anchor changed.

## Testing & Quality
- **Unit (source-level, new `tests/unit/test_thinking_no_scroll.py`,
  repo source-pin pattern):** `styles.css` carries
  `details.thinking .thinking-text` with `overflow-y: hidden` and
  `max-height: 320px`; the phase-17 pin line
  (`textEl.scrollTop = textEl.scrollHeight`) still present in `app.js`
  (the live-tail mechanism must not be lost).
- **Integration:** none (no `app/` changes).
- **Coverage:** frontend-only; the >90% `app/` gate is unaffected.
- **E2E:** `tests/e2e/test_thinking_no_scroll.py` (task 02), green
  **in isolation** (prereq `podman compose up -d db`).
- **Lint/types:** `uv run ruff check . && uv run pyright` clean.

## Completion Criteria
- [ ] With a long thinking stream, wheel/mouse-drag/keyboard on
      `.thinking-text` do **not** move it; the visible content is always
      the live tail (`scrollTop === scrollHeight` after each chunk,
      within 1px).
- [ ] Computed style: `overflow-y: hidden`, `max-height: 320px`.
- [ ] A long **answer** bubble still scrolls normally; a restored
      collapsed Thinking block still renders (phase 17 regression).
- [ ] `uv run pytest` green; `uv run pytest --cov=app
      --cov-report=term-missing` ≥ today's number.
- [ ] `uv run pytest tests/e2e/test_thinking_no_scroll.py -v --no-cov`
      green in isolation; regressions green in isolation:
      `test_thinking_display.py`, `test_long_answers.py`.
- [ ] `uv run ruff check . && uv run pyright` clean.
- [ ] UI Structure Check (AGENTS.md rule 5): no new surface; the block
      keeps its focus-visible summary, aria contract, and reduced-motion
      behavior.
- [ ] `.agents/user_stories/thinking-no-scroll.md` exists.
- [ ] One `--no-gpg-sign` commit (below);
      `.agents/phases/todo/21_thinking_no_scroll/` moved to
      `.agents/phases/complete/`.

## Commit
```bash
git add -A .agents/ frontend/ tests/ && git commit --no-gpg-sign -m "fix(ui): thinking window no longer scrolls — live 320px view pinned to the stream tail"
```