# Story: No reply autoscroll

**Phase:** `42_no_reply_autoscroll` · **Source:** `TODO.md` L5 ·
**E2E:** `tests/e2e/test_no_reply_autoscroll.py`

## Bug report (verbatim, `TODO.md` L5)

> "Get rid of the chat reply autoscroll, it's breaking things like
> making it impossible for the user to scroll while a reply generates."

## Narrative

As **a user reading a long answer**, I want full control of the
viewport while Brain replies. Today the page auto-scrolls to follow the
stream (phase 18 "follow-the-bottom"): while I'm in the 200px
near-bottom band the page is yanked down on every thinking / tool /
delta frame, which fights my own scrolling mid-answer. The reply
autoscroll is being **removed** — the page only scrolls when I
explicitly cause it.

- **Given** a reply is streaming (thinking, tool calls, or answer text)
- **When** I scroll up to read earlier context
- **Then** the viewport stays exactly where I put it for the rest of the
  turn — no frame yanks it back.

## Acceptance criteria
1. **No streaming autoscroll:** during a long thinking stream, a tool
   call, and a long answer, the page never auto-scrolls — sampled
   `window.scrollY` is stable (within 1px) across frames while the
   viewport is away from the bottom.
2. **Submit reveals my message:** sending a question while scrolled up
   still scrolls the viewport down so my own message is visible
   (user-initiated — kept by owner decision).
3. **Restore landing kept:** reloading a persisted conversation
   (phase 14) still lands one-shot on the latest message.
4. **The phase-18 gate is gone:** `NEAR_BOTTOM_PX` /
   `isNearBottom()` and the per-frame `scrollReveal` calls in the
   thinking / tool / delta handlers are removed from `app.js`; the unit
   pin (`tests/unit/test_frontend_scroll.py`) is rewritten for the new
   contract (scrolls happen only on submit + restore landing).
5. **Everything else unchanged:** the thinking window's *internal*
   bottom-pin (phase 17 — `textEl.scrollTop`, not the page) is untouched
   in this phase (phase 43 reworks it separately); message rendering,
   persistence, UI states, and the 120 s guard are unchanged.

## Owner-confirmed (2026-08-27, roadmap A1)
1. **"Reply autoscroll" = the phase-18 follow-the-bottom auto-follow on
   thinking / tool / delta frames.** Removed.
2. **Kept:** scroll-on-submit (reveal the user's own message) and the
   one-shot restore landing on page load.

## UI Visualization & Structure
- **The functional change is in `frontend/assets/app.js` only:**
  - delete `export const NEAR_BOTTOM_PX = 200`, `isNearBottom()`, and
    the `force`-optional gating in `scrollReveal` — the helper becomes an
    unconditional `scrollIntoView` (still smooth, still still under
    `prefers-reduced-motion` via the existing `SCROLL` constant);
  - `addMessage(...)` gains an explicit "scroll" intent: the **user
    submit** path scrolls (shows my message), brain bubble creation and
    the typing indicator do **not**;
  - the thinking / tool / delta handlers drop their `scrollReveal(wrap)`
    calls (the thinking handler keeps its `textEl.scrollTop` window-pin —
    phase 17, reworked in phase 43);
  - the phase-14 restore landing keeps its one-shot forced scroll;
  - module docstrings updated (the phase-18 contract block is replaced
    by the new "no reply autoscroll (owner direction 2026-08-27)"
    contract).
- **Non-goals:** no new UI element, no "↓ new content" pill (the owner
  wants silence, not a substitute affordance), no change to the
  composer / safe-area layout.

## Playwright Mapping Rule
**Test Scenario → `tests/e2e/test_no_reply_autoscroll.py`** (mock LLM;
DB up; the phase-18 suite `tests/e2e/test_follow_bottom_scroll.py` is
**deleted** in this phase — its behavior is intentionally removed):
1. `test_no_autoscroll_during_long_answer` — `LONG_ANSWER_TRIGGER`
   question; once the answer starts, scroll the viewport up ~2× the
   answer height; sample `window.scrollY` across ≥10 streaming frames:
   stable within 1px; after `done` the viewport is still where it was.
2. `test_no_autoscroll_during_thinking` — `THINKING_TRIGGER` question;
   scroll up during the ~4.5 s thinking stream; the viewport stays
   pinned (no per-chunk page follow).
3. `test_submit_reveals_user_message` — scroll to the very top of a
   populated conversation, send a question; the viewport ends with the
   user's message visible (bottom in view).
4. `test_restore_landing_one_shot` (phase 14 regression) — settle a
   conversation, reload; the page lands on the latest message one-shot
   and stays there while no stream is active.
5. `test_answer_content_intact` (regression) — the long answer streams
   to completion with sources and (for thinking) the collapsed block,
   persisted and restorable.