# Task 01 — Sticky Bottom Composer

**Phase:** `52_pinned_composer` · **Source:** `TODO.md:3` — "The message input text box needs to be pinned to the bottom of the screen so it doesn't \"run away\" from the user as they try to click \"stop\""
**Story:** n/a (TODO-derived)

## Objective
The composer stays at the bottom of the screen — resting there on a short page and pinned there at every scroll position of an over-viewport page — with CSS-only changes inside the existing chat column.

## Revision (owner, 2026-08-30) — this task was NOT done the first time
The first pass applied only `position: sticky; bottom: env(safe-area-inset-`
`bottom)` to `.composer`, and the browser disproved it: on an empty/short
chat the input rested just under the empty state (~57% of the viewport)
with a dead band all the way down to the footer. `position: sticky` can
only pull a box UP to the scrollport's bottom edge; it never pushes a box
DOWN to meet it, so on a page that does not overflow it does nothing — the
old E2E contract only ever exercised an overflowing conversation, which is
why the half-fix passed. Both rules below are now in place and both are
source-pinned in `tests/unit/test_pinned_composer.py`.

## Work
1. `frontend/assets/styles.css` — TWO rules, both required:
   a. `.messages { flex: 1 1 auto; }` (Main-frame block) — the message list
      absorbs the free space of a short page, so the composer's resting
      in-flow position is the bottom of the full-height column
      (`body{min-height:100dvh}` → `.app-main{flex:1}` → `.chat-shell{flex:1}`).
      `flex-basis` stays `auto` (a `0` basis would size the list below its
      content once the conversation overflows and let bubbles overlap the
      box); no `height` cap, no `overflow` — the document stays the scroller.
   b. `.composer { position: sticky; bottom: env(safe-area-inset-bottom, 0); }`
      (`/* ---------- Composer ---------- */` block, ~L1133) — takes over the
      moment the conversation overflows. The page scrolls at the document level and `.chat-shell` is the composer's containing column, so the box sticks to the viewport's bottom edge (offset by the mobile safe-area inset) while `.messages` scrolls behind it; at the document bottom it settles back into its normal flow position above the footer. Keep the existing solid `background: var(--surface)`, border, radius and `box-shadow: var(--shadow)` — messages must never show through the pinned box.
2. `frontend/index.html` — verify NO change needed: the composer is already the LAST child of `.chat-shell` (the sticky context), and the `#message-input` / `#send-btn` / `#send-status` markup is untouched.
3. Do NOT touch `frontend/assets/app.js` — the pin must not add any scroll call site (phase-42 invariant; the one page scroll in the file stays `scrollReveal`).
4. `tests/unit/test_pinned_composer.py` (new, house pin style — see `tests/unit/test_frontend_scroll.py`): assert `styles.css` declares `position: sticky` AND a `bottom:` offset on `.composer` (the sticky-bottom pair, matched inside the `.composer` rule); assert `app.js` is unchanged in its scroll surface (the phase-42 single-`scrollReveal` pin still holds — reuse the same assertion approach `test_no_reply_autoscroll.py`'s companion pins use).
- ASSUMPTION (revised): `bottom: env(safe-area-inset-bottom, 0)` — the
  notch-aware inset with an explicit `0` fallback; `env()`-only (the first
  pass) degrades to `auto`, i.e. no pin, where the function is unsupported.
- ASSUMPTION: no `z-index` added — DOM order already stacks the composer above `.messages`; the sticky header (z 20) and doc modal (z 1000) are unaffected.

## Testing & Quality
- Unit: `tests/unit/test_pinned_composer.py` (the pins above) — `uv run pytest tests/unit/test_pinned_composer.py -v` green.
- Coverage: **>90%** — no `app/` change; the gate stays green.

## Completion Criteria
- [x] `.messages` carries the `flex-grow` and `.composer` carries `position: sticky` + the `bottom` safe-area offset with the `0` fallback; the `frontend/` diff contains no JS and no DOM change.
- [x] `uv run pytest` green (1019 passed, TOTAL 99%); `uv run ruff check . && uv run pyright` clean.