diff --git a/.agent/phases/todo/52_pinned_composer/01_sticky_composer.md b/.agent/phases/complete/52_pinned_composer/01_sticky_composer.md
similarity index 100%
rename from .agent/phases/todo/52_pinned_composer/01_sticky_composer.md
rename to .agent/phases/complete/52_pinned_composer/01_sticky_composer.md
diff --git a/frontend/assets/styles.css b/frontend/assets/styles.css
index d745d8f..98987aa 100644
--- a/frontend/assets/styles.css
+++ b/frontend/assets/styles.css
@@ -1130,10 +1130,33 @@ details.thinking .thinking-text ul { margin: 0 0 0.5rem; }
 .suggestion-chip:active { transform: scale(0.98); }
 
 /* ---------- Composer ---------- */
+/* Phase 52 (2026-08-30, TODO.md L3): the composer is PINNED to the
+   viewport bottom. The page scrolls at the document level and
+   `.chat-shell` (the centered 46rem column) is the composer's sticky
+   containing block, so the box sticks to the bottom edge of the
+   viewport at every scroll position and settles back into its normal
+   flow position (above the footer) once the document bottom is
+   reached. Before this, a long conversation put the composer below the
+   fold while phase 42 forbids auto-scrolling a streaming reply — the
+   Stop control could literally not be reached ("it runs away from the
+   user as they try to click stop").
+   CSS-only by design: no DOM change, no JS, and NO new scroll call
+   site (the phase-42 never-auto-scroll contract stays intact —
+   `scrollReveal` is still the one page scroll in app.js). The offset
+   is the notch-aware safe-area inset (`env()` resolves to 0 on
+   desktop, so the box sits flush with the viewport bottom; ≤640px keeps
+   its own `main { padding-bottom: env(safe-area-inset-bottom) }`).
+   The solid `--surface` background, border, radius and shadow are kept
+   so messages scrolling behind the pinned box never show through it,
+   and no z-index is added: DOM order already paints the composer over
+   `.messages`, it never overlaps the sticky header (z 20) and stays
+   under the z-1000 document modal. */
 .composer {
   display: flex;
   align-items: flex-end;
   gap: 0.6rem;
+  position: sticky;
+  bottom: env(safe-area-inset-bottom);
   background: var(--surface);
   border: 1px solid var(--line);
   border-radius: var(--radius);
diff --git a/tests/e2e/test_pinned_composer.py b/tests/e2e/test_pinned_composer.py
new file mode 100644
index 0000000..046a850
--- /dev/null
+++ b/tests/e2e/test_pinned_composer.py
@@ -0,0 +1,554 @@
+"""Phase 52 E2E (Playwright): the composer never runs away from the user.
+
+Source: ``TODO.md`` L3 — "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\"" (no user story file — TODO-derived phase).
+Run in isolation (DB must be up: ``podman compose up -d db``):
+
+    uv run pytest tests/e2e/test_pinned_composer.py -v --no-cov
+
+Mock-only, no admin login (chat is public). The owner-locked contract
+(2026-08-30) under test — the CSS pin (``position: sticky;
+bottom: env(safe-area-inset-bottom)`` on ``.composer``, phase 52 task 01)
+observed in a real browser:
+
+* with an over-viewport conversation the composer box is fully inside the
+  viewport, flush with its bottom edge, at EVERY scroll position — the
+  reader never has to scroll to find the input;
+* the original bug scenario: mid-turn, scrolled up reading earlier
+  content, the Stop control (``#send-btn.is-stop``, phase 48) is visible
+  and clickable WITHOUT scrolling, the click settles the turn (partial
+  kept + ``.stopped-note``, no error banner), it moves the viewport not
+  at all (the phase-42 never-auto-scroll contract — the pin adds no scroll
+  call site), and a reload restores the ``stopped`` record through the
+  normal ``bor.chat.v1`` path;
+* on an empty/short chat the composer renders in its normal flow position
+  — the pin does not float it over the footer (``.app-footer`` stays in
+  flow below it), and at the document bottom the box has settled back into
+  flow above the footer instead of hovering on the viewport edge;
+* ≤640px the pin holds too (the composer is reachable, ≥44px Send target)
+  and stays UNDER the sticky header — no z-index was added, so the pinned
+  box can never cover the header or the hamburger dropdown (phase 46).
+
+Determinism: the mock's grounded answers quote the question and end in the
+``Deterministic mock answer for E2E`` marker; the long answer (``write a
+long answer`` trigger, phase 11) streams ~900 words over ~8 s at 12 chars /
+0.02 s — the in-flight window the Stop click rides — with a unique final
+line (``LONG-ANSWER-END``) absent from any partial. Overflow is built from
+real UI turns (no fixture, no DOM injection): 6 short turns put ~1.5k px
+of conversation into the 800px viewport.
+
+Test → story mapping (Playwright Mapping Rule):
+1. ``test_composer_pinned_at_every_scroll_position``
+2. ``test_stop_is_reachable_from_scrolled_up`` — the TODO.md L3 scenario
+3. ``test_empty_chat_composer_sits_in_normal_flow``
+4. ``test_pin_holds_on_mobile_under_the_header`` — ≤640px stacking
+"""
+from __future__ import annotations
+
+import asyncio
+import json
+import re
+import time
+from collections.abc import Iterator
+from pathlib import Path
+from threading import Thread
+from typing import Any
+
+import pytest
+from playwright.sync_api import Browser, Page, ViewportSize, expect
+from sqlalchemy import text
+
+from app.config import Settings
+from app.db import SessionLocal
+from app.rag.importer import ImportSummary, import_sources
+from app.rag.llm import LLMClient
+
+REPO = Path(__file__).resolve().parents[2]
+FIXTURES = REPO / "tests" / "fixtures" / "docs"
+
+#: Six short grounded questions (the house phrasing + on-topic variants —
+#: the honesty gate is HIGH for all of them, so every turn renders an
+#: answer bubble + source chips). Six turns overflow the 800px viewport by
+#: a wide margin; the tests assert the overflow, so the count can never
+#: silently stop being enough.
+SHORT_QUESTIONS = (
+    "How is my Kubernetes cluster set up?",
+    "How do my backups work?",
+    "How did I install gitlab?",
+    "How does my homelab networking work?",
+    "What is in the new-service deployment?",
+    "What scripts do I have in the homelab?",
+)
+#: The phase-11 long-answer trigger: ~900 words ≈ 8 s of streaming — the
+#: in-flight window the scrolled-up Stop click rides.
+LONG_QUESTION = "How is my Kubernetes cluster set up? write a long answer"
+#: The long answer's unique final line — absent from any partial.
+LONG_ANSWER_END = "LONG-ANSWER-END"
+MOCK_ANSWER_MARKER = "Deterministic mock answer for E2E"
+STORAGE_KEY = "bor.chat.v1"
+
+#: The conftest ``page`` viewport is 1280×800; the story's phone viewport
+#: is the phase-46 one.
+MOBILE: ViewportSize = {"width": 375, "height": 812}
+
+#: Tolerance for "flush with the viewport bottom": `env(safe-area-inset-
+#: bottom)` resolves to 0 on a desktop/notched-free viewport, so the only
+#: slack is sub-pixel rounding. A notched device would shift the box up by
+#: the inset — the same few px of headroom the assertion allows.
+FLUSH_PX = 4
+#: The sticky header's band (phase 12 pins it at 64px) — the pinned
+#: composer must never enter it.
+HEADER_PX = 64
+
+# The typing indicator is itself a .msg.brain — exclude its bubble.
+ANSWER = ".msg.brain .bubble:not(.typing)"
+
+
+# ---------------------------------------------------------------------------
+# KB seeding (same pattern as the phase 42/48/49 story suites)
+# ---------------------------------------------------------------------------
+
+
+async def _import_fixtures(mock_port: int) -> ImportSummary:
+    kwargs: dict[str, Any] = {"_env_file": None, "llm_base_url": f"http://127.0.0.1:{mock_port}/v1"}
+    settings = Settings(**kwargs)  # pyright: ignore[reportCallIssue]
+    return await import_sources([FIXTURES], LLMClient(settings))
+
+
+def _run_in_thread(coro: Any) -> Any:
+    """Run a coroutine on a worker thread.
+
+    Playwright's sync API keeps an asyncio loop running on the test
+    thread, so ``asyncio.run`` cannot be called directly from a test body.
+    """
+    box: dict[str, Any] = {}
+
+    def runner() -> None:
+        try:
+            box["value"] = asyncio.run(coro)
+        except BaseException as e:  # noqa: BLE001 — re-raised on the test thread
+            box["error"] = e
+
+    t = Thread(target=runner)
+    t.start()
+    t.join()
+    if "error" in box:
+        raise box["error"]
+    return box["value"]
+
+
+def _reset_db(mock_port: int, seed: bool) -> ImportSummary | None:
+    """Truncate the KB (and query log), then optionally re-import fixtures."""
+    with SessionLocal() as db:
+        db.execute(text("TRUNCATE chunks, documents, query_log"))
+        db.commit()
+    if not seed:
+        return None
+    return _run_in_thread(_import_fixtures(mock_port))
+
+
+@pytest.fixture()
+def seeded_kb(mock_llm: int, db_ready: None) -> Iterator[None]:
+    """A fresh KB seeded from ``tests/fixtures/docs`` (13 docs, A9 formats),
+    truncated again on teardown. ``db_ready`` (conftest) skips with clear
+    instructions when Postgres is down."""
+    summary = _reset_db(mock_llm, seed=True)
+    assert summary is not None and summary.added == 13
+    yield
+    _reset_db(mock_llm, seed=False)
+
+
+# ---------------------------------------------------------------------------
+# Measurement + flow helpers
+# ---------------------------------------------------------------------------
+
+
+def scroll_state(page: Page) -> dict[str, float]:
+    """The document scroller's state (there is no inner scroll container)."""
+    return page.evaluate(
+        "() => ({ y: window.scrollY, "
+        "sh: document.documentElement.scrollHeight, "
+        "ch: window.innerHeight })"
+    )
+
+
+def wait_settled(page: Page, timeout: int = 30_000) -> None:
+    """The turn is over: the label is back to "Send" (phase 48 — the
+    in-flight state is the enabled Stop control, so the label carries the
+    wait; the explicit timeout covers the mock's ~8 s long turn)."""
+    expect(page.locator("#send-btn")).to_be_enabled(timeout=timeout)
+    expect(page.locator("#send-label")).to_have_text("Send", timeout=timeout)
+
+
+def submit(page: Page, question: str) -> None:
+    """Submit through the composer (the real-user flow)."""
+    page.fill("#message-input", question)
+    page.click("#send-btn")
+    expect(page.locator(".msg.user .bubble").last).to_contain_text(question)
+
+
+def build_conversation(page: Page, n: int = 6) -> None:
+    """Settle ``n`` short grounded turns through the UI (the overflow is
+    real conversation content, not injected DOM)."""
+    for question in SHORT_QUESTIONS[:n]:
+        submit(page, question)
+        wait_settled(page)
+
+
+def box_of(page: Page, selector: str) -> dict[str, float]:
+    """The selector's viewport-relative box as a plain dict.
+
+    Measured WITHOUT scrolling (``bounding_box`` never scrolls the page), so
+    the answer is "where the box sits where the user left it" — exactly the
+    question this story asks. Playwright hands back a FloatRect; the helpers
+    below speak ``dict[str, float]``.
+    """
+    rect = page.locator(selector).bounding_box()
+    assert rect is not None, f"{selector} must be rendered (no bounding box)"
+    return {
+        "x": rect["x"],
+        "y": rect["y"],
+        "width": rect["width"],
+        "height": rect["height"],
+    }
+
+
+def composer_box(page: Page) -> dict[str, float]:
+    return box_of(page, "#composer")
+
+
+def assert_flush_with_viewport_bottom(page: Page) -> dict[str, float]:
+    """The composer box is FULLY inside the viewport with its bottom edge
+    at the viewport bottom (± ``FLUSH_PX`` — the safe-area inset / sub-pixel
+    rounding). This is the whole story: the input is where the user's
+    pointer already is, at every scroll position."""
+    box = composer_box(page)
+    ch = scroll_state(page)["ch"]
+    assert box["y"] >= -FLUSH_PX, (
+        f"the pinned composer is clipped at the TOP of the viewport "
+        f"(y={box['y']:.1f}, viewport={ch:.0f})"
+    )
+    assert box["y"] + box["height"] <= ch + FLUSH_PX, (
+        f"the composer runs BELOW the viewport bottom — it ran away "
+        f"(bottom={box['y'] + box['height']:.1f}, viewport={ch:.0f})"
+    )
+    assert box["y"] + box["height"] >= ch - FLUSH_PX, (
+        f"the composer is not pinned to the bottom edge "
+        f"(bottom={box['y'] + box['height']:.1f}, viewport={ch:.0f})"
+    )
+    return box
+
+
+def assert_no_overlap(a: dict[str, float], b: dict[str, float]) -> None:
+    """Two bounding boxes must not overlap vertically."""
+    assert a["y"] + a["height"] <= b["y"] + FLUSH_PX, (
+        f"boxes overlap: {a} vs {b}"
+    )
+
+
+def _assert_no_error_banner(page: Page) -> None:
+    """A stop settles to idle through the same finally path — never the red
+    role=alert error banner (phase 48 contract)."""
+    banner = page.locator("#kb-banner")
+    expect(banner).to_be_hidden()
+    expect(banner).not_to_have_attribute("role", "alert")
+    expect(banner).not_to_have_class(re.compile(r"is-error"))
+
+
+def _stored_parsed(page: Page) -> dict[str, Any]:
+    raw = page.evaluate(f"() => localStorage.getItem('{STORAGE_KEY}')")
+    assert raw is not None, "the conversation key must exist in localStorage"
+    return json.loads(raw)
+
+
+# ---------------------------------------------------------------------------
+# 1. Pinned while reading: over-viewport conversation, every scroll
+#    position keeps the composer inside the viewport at the bottom edge,
+#    and at the document bottom it settles back into flow above the footer
+# ---------------------------------------------------------------------------
+
+
+def test_composer_pinned_at_every_scroll_position(
+    page: Page, app_url: str, seeded_kb: None
+) -> None:
+    page.set_default_timeout(30_000)
+    page.goto(app_url)
+
+    build_conversation(page, n=len(SHORT_QUESTIONS))
+
+    # The story's precondition: a conversation longer than the viewport.
+    state = scroll_state(page)
+    assert state["sh"] > state["ch"] + 200, (
+        f"the conversation must overflow the viewport (sh={state['sh']:.0f}, "
+        f"ch={state['ch']:.0f}) — more turns are needed"
+    )
+
+    # Scrolled to the very top (the "reading the beginning" position): the
+    # pinned box sits at the bottom edge of the viewport, fully visible.
+    page.evaluate("() => window.scrollTo(0, 0)")
+    assert scroll_state(page)["y"] <= 1
+    assert_flush_with_viewport_bottom(page)
+
+    # And at every stop through the sticky range — the range is bounded by
+    # the composer's CONTAINING BLOCK (`.chat-shell`): the pin holds while
+    # that column's bottom edge sits below the viewport bottom, which is
+    # every reading position above the last screenful of conversation.
+    limits = page.evaluate(
+        "() => ({ shellBottom: document.querySelector('.chat-shell')"
+        ".getBoundingClientRect().bottom + window.scrollY, "
+        "sh: document.documentElement.scrollHeight, ch: window.innerHeight })"
+    )
+    max_scroll = limits["sh"] - limits["ch"]
+    pin_limit = limits["shellBottom"] - limits["ch"]
+    assert pin_limit > 200, (
+        f"the conversation must keep the composer inside its sticky range "
+        f"for at least a screen of scrolling (limit={pin_limit:.0f})"
+    )
+    assert max_scroll > pin_limit, (
+        "the document bottom must leave the sticky range, so the settle "
+        "back into flow is covered too"
+    )
+    for y in (0, pin_limit * 0.25, pin_limit * 0.5, pin_limit * 0.75, pin_limit - 1):
+        page.evaluate("y => window.scrollTo(0, y)", y)
+        assert abs(scroll_state(page)["y"] - y) <= 1, f"the test scroll to {y} must land"
+        assert_flush_with_viewport_bottom(page)
+
+    # Settled at the document bottom: the sticky box has returned to its
+    # NORMAL FLOW position — above the footer, not hovering over it (the
+    # pin must not float the composer over the footer).
+    page.evaluate("() => window.scrollTo(0, document.documentElement.scrollHeight)")
+    assert abs(scroll_state(page)["y"] - max_scroll) <= 1, "must land at the document bottom"
+    footer_box = box_of(page, ".app-footer")  # the footer must be rendered
+    composer = composer_box(page)
+    assert_no_overlap(composer, footer_box)
+    assert composer["y"] + composer["height"] < scroll_state(page)["ch"] - FLUSH_PX, (
+        "at the document bottom the composer settles into flow — it is no "
+        "longer glued to the viewport edge"
+    )
+
+    # The conversation itself is intact (6 turns, all answers landed) — the
+    # pin changed the geometry, not the content.
+    expect(page.locator(".msg.user .bubble")).to_have_count(len(SHORT_QUESTIONS))
+    expect(page.locator(".msg.brain .bubble")).to_have_count(len(SHORT_QUESTIONS))
+    expect(page.locator(".msg.brain .bubble").last).to_contain_text(MOCK_ANSWER_MARKER)
+
+
+# ---------------------------------------------------------------------------
+# 2. The TODO.md L3 scenario: mid-turn, scrolled up reading earlier
+#    content, Stop is visible and clickable without scrolling — and the
+#    click neither moves the viewport nor breaks the phase-48 contract
+# ---------------------------------------------------------------------------
+
+
+def test_stop_is_reachable_from_scrolled_up(
+    page: Page, app_url: str, seeded_kb: None
+) -> None:
+    page.set_default_timeout(30_000)
+    page.goto(app_url)
+
+    # Turn 1 (settled long answer) overflows the viewport — the user now
+    # has earlier content to read while turn 2 streams.
+    submit(page, LONG_QUESTION)
+    wait_settled(page)
+    state = scroll_state(page)
+    assert state["sh"] > state["ch"] + 200, "a long answer must make the document scrollable"
+
+    # Turn 2 streams (~8 s). Wait until it is observably in flight — the
+    # SECOND answer bubble exists and carries deltas (`.last` alone would
+    # still resolve to turn 1's finished bubble: the typing indicator is
+    # `.bubble.typing` and excluded from ANSWER).
+    submit(page, LONG_QUESTION)
+    page.wait_for_function(
+        "() => { const els = document.querySelectorAll('.msg.brain .bubble:not(.typing)');"
+        " return els.length >= 2 && els[els.length - 1].innerText.length > 80; }",
+        timeout=30_000,
+    )
+    answer = page.locator(ANSWER).nth(1)
+    expect(page.locator("#send-label")).to_have_text("Stop", timeout=10_000)
+
+    # The user scrolls UP to read earlier content. Phase 42 leaves them
+    # there — the app never follows the stream.
+    page.evaluate("() => window.scrollTo(0, 0)")
+    y0 = scroll_state(page)["y"]
+    assert y0 <= 1, "the test scroll to the top must land"
+    # The scrolled-up position is inside the composer's sticky range (the
+    # long answer + its question still extend past this screenful) — the
+    # premise of the owner's scenario.
+    assert page.evaluate(
+        "() => document.querySelector('.chat-shell').getBoundingClientRect().bottom"
+    ) > scroll_state(page)["ch"]
+
+    # ... and the Stop control is right there, WITHOUT scrolling: it is
+    # the enabled `.is-stop` button inside the pinned composer, fully
+    # inside the viewport. (Before phase 52 the composer sat below the
+    # fold here — the "runs away as they try to click stop" bug.)
+    btn = page.locator("#send-btn")
+    expect(page.locator("#send-label")).to_have_text("Stop")
+    expect(btn).to_be_enabled()
+    expect(btn).to_have_class(re.compile(r"is-stop"))
+    expect(btn).to_be_visible()
+    btn_box = box_of(page, "#send-btn")
+    ch = scroll_state(page)["ch"]
+    assert btn_box["y"] >= 0 and btn_box["y"] + btn_box["height"] <= ch, (
+        f"Stop is off-screen while the user reads up-page "
+        f"(box={btn_box}, viewport={ch:.0f})"
+    )
+    # Clicking must not need a scroll: the click lands on Stop itself
+    # (Playwright hits the element at its own position, no scrolling).
+    assert_flush_with_viewport_bottom(page)
+
+    # Stop it — from the scrolled-up position, no scrolling involved.
+    btn.click()
+
+    # The viewport did not move: the pin adds no scroll call site and the
+    # stop path never scrolls (phase 42, sampled across the settle).
+    samples = [scroll_state(page)["y"]]
+    for _ in range(4):
+        time.sleep(0.25)
+        samples.append(scroll_state(page)["y"])
+    assert max(samples) - min(samples) <= 1, (
+        f"the Stop click moved the viewport ({samples}) — the reader is "
+        "yanked away from the content they were reading"
+    )
+
+    # Settled to idle with the phase-48 contract intact.
+    wait_settled(page, timeout=10_000)
+    expect(btn).not_to_have_class(re.compile(r"is-stop"))
+    _assert_no_error_banner(page)
+    expect(page.locator("#send-status")).to_contain_text("Answer stopped.")
+
+    # The partial is kept on screen with the "Stopped" marker; the full
+    # long answer never arrived.
+    stopped_text = answer.inner_text()
+    assert stopped_text.strip()
+    assert "Step 1:" in stopped_text
+    assert LONG_ANSWER_END not in stopped_text
+    expect(page.locator(".msg.brain .stopped-note")).to_have_count(1)
+    expect(page.locator(".msg.brain .stopped-note").first).to_contain_text("Stopped")
+
+    # ... and it persists through the normal bor.chat.v1 path: a fresh
+    # load restores the stopped record (the reader sees it wherever they
+    # were reading).
+    stored = _stored_parsed(page)
+    assert [m["who"] for m in stored["messages"]] == [
+        "user",
+        "brain",
+        "user",
+        "brain",
+    ]
+    last = stored["messages"][-1]
+    assert last["stopped"] is True
+    assert LONG_ANSWER_END not in last["text"]
+
+    page.reload()
+    expect(page.locator(".msg.user .bubble")).to_have_count(2)
+    bubble = page.locator(ANSWER)
+    expect(bubble).to_have_count(2)
+    expect(bubble.nth(1)).to_contain_text("Step 1:")
+    expect(page.locator(".msg.brain .stopped-note")).to_have_count(1)
+    expect(page.locator(".msg.brain .stopped-note").first).to_contain_text("Stopped")
+    expect(bubble.nth(1)).not_to_contain_text(LONG_ANSWER_END)
+    wait_settled(page, timeout=10_000)
+    # The restored page keeps the pin too (the composer is still the
+    # last, sticky child of the column).
+    assert composer_box(page)["height"] > 0
+    assert page.evaluate(
+        "() => getComputedStyle(document.querySelector('#composer')).position"
+    ) == "sticky"
+
+
+# ---------------------------------------------------------------------------
+# 3. Natural bottom: an empty/short chat keeps the composer in its flow
+#    position — the pin does not float it over the footer or shift the page
+# ---------------------------------------------------------------------------
+
+
+def test_empty_chat_composer_sits_in_normal_flow(
+    page: Page, app_url: str, seeded_kb: None
+) -> None:
+    page.set_default_timeout(30_000)
+    page.goto(app_url)
+
+    # A fresh visitor: the empty state, nothing to scroll.
+    expect(page.locator("#empty-state")).to_be_visible()
+    state = scroll_state(page)
+    assert state["sh"] <= state["ch"] + 1, (
+        "an empty chat must not be scrollable — there is nothing to pin against"
+    )
+
+    composer = composer_box(page)
+    assert composer["y"] + composer["height"] <= state["ch"] + FLUSH_PX, (
+        "the composer may never hang below the viewport bottom"
+    )
+
+    # The footer is present, in normal flow, BELOW the composer: the sticky
+    # box settles in its own flow slot (last child of .chat-shell) instead
+    # of floating over the footer on a short page.
+    footer = page.locator(".app-footer")
+    expect(footer).to_be_visible()
+    assert_no_overlap(composer, box_of(page, ".app-footer"))
+
+    # A single short turn: still in flow, still nothing to scroll past.
+    submit(page, SHORT_QUESTIONS[0])
+    wait_settled(page)
+    state = scroll_state(page)
+    composer = composer_box(page)
+    assert composer["y"] + composer["height"] <= state["ch"] + FLUSH_PX
+    assert_no_overlap(composer, box_of(page, ".app-footer"))
+    expect(page.locator(ANSWER).last).to_contain_text(MOCK_ANSWER_MARKER)
+
+
+# ---------------------------------------------------------------------------
+# 4. ≤640px: the pin holds on a phone and stays UNDER the sticky header
+#    (no z-index was added, so the pinned box cannot cover the bar or the
+#    hamburger dropdown — phase 46 stacking)
+# ---------------------------------------------------------------------------
+
+
+def test_pin_holds_on_mobile_under_the_header(
+    browser: Browser, app_url: str, seeded_kb: None
+) -> None:
+    page = browser.new_page(viewport=MOBILE)
+    try:
+        page.set_default_timeout(30_000)
+        page.goto(app_url)
+        build_conversation(page, n=4)
+
+        state = scroll_state(page)
+        assert state["sh"] > state["ch"] + 200, (
+            f"four turns must overflow an 812px phone (sh={state['sh']:.0f}, "
+            f"ch={state['ch']:.0f})"
+        )
+
+        page.evaluate("() => window.scrollTo(0, 0)")
+        assert page.evaluate(
+            "() => document.querySelector('.chat-shell').getBoundingClientRect().bottom"
+        ) > scroll_state(page)["ch"], "the phone position must sit in the sticky range"
+        composer = assert_flush_with_viewport_bottom(page)
+
+        # The pinned box never enters the sticky header's band, and the
+        # header stays on top: the composer carries NO z-index, so DOM
+        # order + the header's z 20 keep the bar (and its dropdown) above.
+        assert composer["y"] >= HEADER_PX, (
+            f"the pinned composer overlaps the header band "
+            f"(y={composer['y']:.1f}, header={HEADER_PX}px)"
+        )
+        assert page.evaluate(
+            "() => getComputedStyle(document.querySelector('#composer')).zIndex"
+        ) in {"auto", "0"}, "the composer must not be lifted above the header"
+        header_z = page.evaluate(
+            "() => getComputedStyle(document.querySelector('.app-header')).zIndex"
+        )
+        assert header_z == "20"
+
+        # Reachable touch target: the Send button stays ≥44px on a phone.
+        btn_box = box_of(page, "#send-btn")
+        assert btn_box["height"] >= 44 and btn_box["width"] >= 44
+
+        # The composer is usable there: typing + sending works from the pin.
+        submit(page, SHORT_QUESTIONS[1])
+        wait_settled(page)
+        expect(page.locator(".msg.user .bubble")).to_have_count(5)
+    finally:
+        page.close()
diff --git a/tests/unit/test_pinned_composer.py b/tests/unit/test_pinned_composer.py
new file mode 100644
index 0000000..3b823a1
--- /dev/null
+++ b/tests/unit/test_pinned_composer.py
@@ -0,0 +1,320 @@
+"""Unit: the pinned (sticky-bottom) composer in the static frontend
+(phase 52, owner direction 2026-08-30, TODO.md L3 — "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'").
+
+The chat page scrolls at the DOCUMENT level and `.chat-shell` (the
+centered 46rem column, PLAN §7.1) is the composer's sticky containing
+block, so `position: sticky; bottom: env(safe-area-inset-bottom)` on
+`.composer` pins the box to the viewport's bottom edge at every scroll
+position and lets it settle back into its normal flow position (above
+the footer) once the document bottom is reached. The pin is CSS-only:
+
+* `.composer` carries the sticky pair (``position: sticky`` + the
+  notch-aware ``bottom`` inset) and keeps its solid ``--surface``
+  background, border, radius and shadow — a reply scrolling behind the
+  pinned box must never show through it;
+* NO ``z-index`` is added to the composer (it already paints above
+  ``.messages`` by DOM order, never overlaps the sticky header (z 20)
+  and stays under the z-1000 document modal), and the pin is not undone
+  in the ≤640px media query;
+* the sticky context survives: `.chat-shell` / `.app-main` / `.messages`
+  gain no ``overflow`` and `.messages` gains no inner scroller — the page
+  keeps scrolling at the document level (the phase-42 model);
+* ``index.html`` needs no change: the composer is already the LAST child
+  of `.chat-shell` (the sticky shift range is that column's box) and the
+  `#message-input` / `#send-btn` / `#send-status` markup is untouched;
+* ``app.js`` gains NO page-scroll call site — the phase-42 invariant
+  (``scrollReveal`` is still the single ``window.scrollTo``; no
+  ``scrollIntoView``, no ``window.scrollY``) is re-pinned here so the
+  pin can never silently arrive in JS instead of CSS.
+
+The browser geometry itself is E2E-gated by the story suite
+(tests/e2e/test_pinned_composer.py, task 02); these are source pins in
+the house style (see tests/unit/test_frontend_scroll.py).
+"""
+
+from __future__ import annotations
+
+import re
+from html.parser import HTMLParser
+from pathlib import Path
+
+FRONTEND = Path(__file__).resolve().parents[2] / "frontend"
+STYLES_CSS = FRONTEND / "assets" / "styles.css"
+APP_JS = FRONTEND / "assets" / "app.js"
+INDEX_HTML = FRONTEND / "index.html"
+
+VOID_ELEMENTS = frozenset(
+    {
+        "area",
+        "base",
+        "br",
+        "col",
+        "embed",
+        "hr",
+        "img",
+        "input",
+        "link",
+        "meta",
+        "param",
+        "source",
+        "track",
+        "wbr",
+    }
+)
+
+
+def _css() -> str:
+    assert STYLES_CSS.is_file(), f"missing {STYLES_CSS}"
+    return STYLES_CSS.read_text(encoding="utf-8")
+
+
+def _js() -> str:
+    assert APP_JS.is_file(), f"missing {APP_JS}"
+    return APP_JS.read_text(encoding="utf-8")
+
+
+def _html() -> str:
+    assert INDEX_HTML.is_file(), f"missing {INDEX_HTML}"
+    return INDEX_HTML.read_text(encoding="utf-8")
+
+
+def _rule(css: str, selector: str) -> str:
+    """The body of the rule whose selector line is exactly `selector`
+    (multi-line block) — same slicing style as test_history_page.py."""
+    block = re.search(rf"^{re.escape(selector)} \{{\n([\s\S]*?)\n\}}", css, re.MULTILINE)
+    assert block, f"styles.css must carry a `{selector} {{ … }}` rule"
+    return block.group(1)
+
+
+def _walk(node: dict) -> list[dict]:
+    """Every element in a parsed subtree (depth-first, document order)."""
+    out = [node]
+    for child in node["children"]:
+        out.extend(_walk(child))
+    return out
+
+
+def _mobile_block(css: str) -> str:
+    """The ≤640px media query body (the phase-07 responsive block)."""
+    mobile = re.search(r"@media \(max-width: 640px\) \{([\s\S]*?)\n\}\n", css)
+    assert mobile, "the mobile media query must exist"
+    return mobile.group(1)
+
+
+class _Tree(HTMLParser):
+    """Minimal element tree of index.html — enough to ask "who is the
+    last element child of `.chat-shell`?" without a DOM library."""
+
+    def __init__(self) -> None:
+        super().__init__(convert_charrefs=True)
+        self.root: dict = {"tag": "#root", "attrs": {}, "children": []}
+        self._stack: list[dict] = [self.root]
+
+    def handle_starttag(self, tag: str, attrs: list[tuple[str, str | None]]) -> None:
+        node = {"tag": tag, "attrs": dict(attrs), "children": []}
+        self._stack[-1]["children"].append(node)
+        if tag not in VOID_ELEMENTS:
+            self._stack.append(node)
+
+    def handle_endtag(self, tag: str) -> None:
+        for i in range(len(self._stack) - 1, 0, -1):
+            if self._stack[i]["tag"] == tag:
+                del self._stack[i:]
+                return
+
+    def find(self, classes: str) -> dict:
+        """The first element whose `class` attribute contains `classes`
+        (all of them, space-separated)."""
+        wanted = classes.split()
+        stack = [self.root]
+        while stack:
+            node = stack.pop(0)
+            have = (node["attrs"].get("class") or "").split()
+            if node["tag"] != "#root" and all(c in have for c in wanted):
+                return node
+            stack.extend(node["children"])
+        raise AssertionError(f"index.html must contain .{classes}")
+
+
+def _tree() -> _Tree:
+    tree = _Tree()
+    tree.feed(_html())
+    return tree
+
+
+# ---------- the sticky pin itself ----------
+
+
+def test_composer_is_sticky_bottom() -> None:
+    """`.composer` carries the sticky pair — `position: sticky` AND a
+    `bottom` offset — inside its own rule. `bottom` is the notch-aware
+    safe-area inset (on desktop `env()` resolves to 0, so the box sits
+    flush with the viewport bottom; on a notched phone the composer
+    clears the home indicator instead of hiding under it)."""
+    body = _rule(_css(), ".composer")
+    assert "position: sticky;" in body, (
+        "the composer must be sticky — phase 52 pins it to the viewport "
+        "bottom so Stop is reachable without scrolling (TODO.md L3)"
+    )
+    assert "bottom: env(safe-area-inset-bottom);" in body, (
+        "the sticky offset must be the safe-area inset (the phase-07 "
+        "mobile contract: the composer stays reachable around the notch)"
+    )
+    # `top` would pin it to the wrong edge (and fight the sticky header).
+    assert "top:" not in body, "the composer pins the BOTTOM edge only"
+
+
+def test_composer_stays_opaque_behind_scrolled_messages() -> None:
+    """The pinned box overlaps the message list while the page is
+    scrolled: it keeps its SOLID `--surface` background plus border,
+    radius and shadow (no glass/transparency), so a reply streaming
+    behind it never shows through the input."""
+    body = _rule(_css(), ".composer")
+    assert "background: var(--surface);" in body, (
+        "the composer needs an opaque background — messages must not "
+        "show through the pinned box"
+    )
+    assert "transparent" not in body and "rgb(" not in body, (
+        "no translucent background on the pinned composer"
+    )
+    assert "border: 1px solid var(--line);" in body
+    assert "border-radius: var(--radius);" in body
+    assert "box-shadow: var(--shadow);" in body, (
+        "the elevation shadow separates the pinned box from the content "
+        "scrolling behind it"
+    )
+
+
+def test_no_z_index_added_to_the_composer() -> None:
+    """Locked assumption: NO z-index change. DOM order already paints
+    the composer above `.messages`, it never reaches the sticky header
+    (z 20) at the top and it must stay under the z-1000 document modal."""
+    body = _rule(_css(), ".composer")
+    assert "z-index" not in body, (
+        "the composer stays unlayered — a z-index here could lift it over "
+        "the sticky header (20) or the document modal (1000)"
+    )
+    css = _css()
+    assert re.search(r"\.app-header \{[\s\S]*?z-index: 20;", css), (
+        "the sticky header keeps its layer"
+    )
+    assert re.search(r"^\.doc-modal \{[\s\S]*?z-index: 1000;", css, re.MULTILINE), (
+        "the document modal stays the topmost layer"
+    )
+
+
+def test_mobile_pin_not_undone() -> None:
+    """≤640px: the pin must survive the responsive block — the mobile
+    `.composer` rule only tightens the padding (it must not reset
+    `position` or drop the box out of the sticky model), and the phone
+    keeps its own `main { padding-bottom: env(safe-area-inset-bottom) }`
+    flow padding under the settled box."""
+    mobile = _mobile_block(_css())
+    assert ".composer" in mobile, "the mobile composer rule must remain"
+    mobile_composer = re.search(r"\.composer \{([^}]*)\}", mobile)
+    assert mobile_composer, "the ≤640px block must keep the .composer rule"
+    body = mobile_composer.group(1)
+    assert "padding: 0.5rem;" in body
+    assert "position" not in body, (
+        "the mobile rule must not reset the sticky position"
+    )
+    assert "main { padding-bottom: env(safe-area-inset-bottom, 0); }" in mobile
+
+
+# ---------- the sticky context (no inner scroller, no clipping ancestor) ----------
+
+
+def test_document_level_scroll_is_untouched() -> None:
+    """Sticky resolves against the NEAREST SCROLLING ANCESTOR. The pin
+    relies on that being the document, so no ancestor between the
+    composer and the viewport may become a scroll container: `.chat-shell`
+    and `.app-main` stay overflow-visible and `.messages` gains no inner
+    scroller / height cap (that would move the scroll — and the phase-42
+    contract — into the message list)."""
+    css = _css()
+    for selector in (".chat-shell", ".app-main", ".messages"):
+        body = _rule(css, selector)
+        assert "overflow" not in body, (
+            f"{selector} must not clip/scroll — the composer pins to the "
+            "document viewport, not to an inner scrollport"
+        )
+    messages = _rule(css, ".messages")
+    assert not re.search(r"(?<!min-)\bheight:", messages), (
+        "the message list must not become its own scroller (only the "
+        "phase-01 `min-height` floor is allowed)"
+    )
+
+
+# ---------- index.html: no DOM change needed ----------
+
+
+def test_composer_is_last_child_of_the_chat_shell() -> None:
+    """The composer must remain the LAST element child of `.chat-shell`
+    (the sticky containing block): the sticky shift range is that
+    column's box, so a sibling after the form would carve the range away
+    and re-break the pin. The `#message-input` / `#send-btn` /
+    `#send-status` markup is untouched (the pin ships no DOM change)."""
+    shell = _tree().find("chat-shell")
+    last = shell["children"][-1]
+    assert last["tag"] == "form", (
+        "the composer <form> must stay the last child of .chat-shell"
+    )
+    assert last["attrs"].get("id") == "composer"
+    assert "novalidate" in last["attrs"], (
+        "phase 48: the composer form stays `novalidate` (a `required` "
+        "input would swallow the Stop click)"
+    )
+    ids = {
+        node["attrs"].get("id")
+        for node in _walk(last)
+        if node["attrs"].get("id")
+    }
+    assert {"composer", "message-input", "send-btn", "send-status"} <= ids, (
+        "the composer markup contract ids must all be present (task 02 E2E "
+        "clicks Stop from this pinned box)"
+    )
+
+
+# ---------- app.js: the pin adds no page scroll (phase-42 invariant) ----------
+
+
+def test_app_js_gains_no_page_scroll_call_site() -> None:
+    """The phase-42 never-auto-scroll contract, re-pinned for phase 52:
+    the composer pin is CSS-only, so `app.js`'s scroll surface is
+    byte-for-byte the phase-42 one — `scrollReveal`'s single
+    `window.scrollTo` is still the ONLY page scroll, no `scrollIntoView`
+    came back, nothing measures `window.scrollY`, and the only other
+    scroll is the thinking window's own bottom pin (`textEl.scrollTop`,
+    a block-internal clip, not the page). A JS-scrolled "pin" (an
+    IntersectionObserver / sticky polyfill / scroll listener) would trip
+    one of these assertions."""
+    js = _js()
+    assert js.count("window.scrollTo(") == 1, "exactly one page scroll may exist"
+    assert js.find("window.scrollTo(") > js.find("function scrollReveal"), (
+        "the one page scroll must live inside scrollReveal"
+    )
+    assert js.count(".scrollIntoView(") == 0
+    assert "window.scrollY" not in js
+    assert "addEventListener(\"scroll\"" not in js, (
+        "the pin must not install a scroll listener"
+    )
+    assert "IntersectionObserver" not in js, (
+        "no JS sticky polyfill — the pin is `position: sticky`"
+    )
+    assert js.count("textEl.scrollTop = textEl.scrollHeight") == 1, (
+        "the thinking window's internal pin stays, and stays the only one"
+    )
+
+
+def test_turn_end_focus_still_uses_preventscroll() -> None:
+    """The turn-end focus-back lands on the pinned composer every turn —
+    with the box now sticky it must still never move the viewport
+    (`focus({ preventScroll: true })`, phase 42), or every finished turn
+    would yank the reader to the bottom."""
+    js = _js()
+    idx = js.find("// done | error | stop → idle: always settle, always focus back")
+    assert idx != -1, "the turn's finally block must exist"
+    block = js[idx : js.find("\n}", idx)]
+    assert "input.focus({ preventScroll: true })" in block