"""Unit: the pinned 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'"; owner revision the same day — the box had to
be at the bottom of the screen on an EMPTY chat too, which sticky alone
cannot do).

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. The pin is therefore TWO rules, and both are pinned here:

* `.messages { flex: 1 1 auto }` absorbs the free space of a short page,
  so the composer's RESTING (in-flow) position is already the bottom of
  the full-height column — `position: sticky` can only pull a box UP
  toward the viewport's bottom edge, it can never push one DOWN to meet
  it, so without the grow the empty/short chat left the input mid-screen
  with a dead band under it (the reported bug);
* `.composer { position: sticky; bottom: env(safe-area-inset-bottom, 0) }`
  takes over the moment the conversation overflows the viewport and keeps
  the box — and the Stop control inside it — glued to the viewport's
  bottom edge at every scroll position, settling back into flow (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, with the explicit ``0`` fallback) 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;
* 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 with an explicit `0` fallback (it resolves to 0 on a
    desktop viewport, so the box sits flush with the viewport bottom; on a
    notched phone the composer clears the home indicator instead of hiding
    under it — and a browser without `env()` support still gets 0)."""
    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, 0);" in body, (
        "the sticky offset must be the safe-area inset with a 0 fallback "
        "(the phase-07 mobile contract: the composer stays reachable "
        "around the notch, and never falls back to `auto` = no pin)"
    )
    # `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_message_list_absorbs_the_short_page_space() -> None:
    """The other half of the pin — and the half phase 52 originally missed.

    `position: sticky` never pushes a box DOWN to the viewport's bottom
    edge, so on an empty/short chat (no free space absorbed) the composer
    sat right under the empty state, mid-screen. `.messages` must GROW to
    take up that space, which puts the composer's resting position at the
    bottom of the full-height column the page already builds
    (`body{min-height:100dvh}` → `.app-main{flex:1}` → `.chat-shell{flex:1}`).
    """
    messages = _rule(_css(), ".messages")
    assert re.search(r"flex:\s*1(\s+1\s+auto)?;", messages), (
        "the message list must grow to fill the short page — otherwise the "
        "pinned composer only works once the conversation overflows and the "
        "empty chat leaves the input mid-screen (owner revision 2026-08-30)"
    )
    # flex-basis must stay `auto`: a `0` basis would size the list BELOW its
    # content when the conversation overflows and let bubbles overlap the box.
    assert not re.search(r"flex:\s*1\s+1\s+0", messages), (
        "flex-basis must stay auto — the list keeps its content height when "
        "the page overflows (no free space to absorb there anyway)"
    )
    # The column the composer sits in must keep stretching to the viewport.
    for selector in (".app-main", ".chat-shell"):
        frame = _rule(_css(), selector)
        assert re.search(r"flex:\s*1[;\s]", frame), (
            f"{selector} must keep growing to the viewport height — the "
            "composer can only rest at the bottom of a full-height column"
        )


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) — `flex-grow` stretches it, "
        "a `height` cap would clip the conversation instead"
    )


# ---------- 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