"""Phase 18 E2E (Playwright, mock-only): the chat follows the bottom.

Story: ``.agent/user_stories/follow-bottom-scroll.md``
Run in isolation (DB must be up: ``podman compose up -d db``):

    uv run pytest tests/e2e/test_follow_bottom_scroll.py -v --no-cov

The follow-the-bottom contract (owner choice 2026-08-23, option 1 — no
"↓ new content" pill): the page auto-scrolls *only while the user is
already pinned at the bottom*; submitting a question reveals the user's
own message; once the user scrolls up, nothing auto-scrolls for the rest
of the turn (thinking or answer); a restored conversation still lands on
the latest message.

MOCK-ONLY suite: the scenarios key off the deterministic mock's
``write a long answer`` trigger (~900 words ≈ 8s of streaming — a wide,
reliable window to scroll away in) and, for scenario 4, the phase-17
``think out loud`` trigger (both fire independently). ``E2E_REAL_LLM=1``
would make the scroll-away windows unpredictable, so it is not supported
here.

Measurement convention: the scroller is the DOCUMENT — there is no inner
scroll container (``body`` is ``min-height: 100dvh``; the page scrolls on
the window). Scroll position is read via ``page.evaluate`` as
``{ y: window.scrollY, sh: document.documentElement.scrollHeight,
ch: window.innerHeight }``; "near bottom" = ``sh - y - ch <= 200``
(mirrors the frontend's ``NEAR_BOTTOM_PX``); scrolling to the top is
``page.evaluate("() => window.scrollTo(0, 0)")``.

Real-user flow: the user submits from the composer — i.e. pinned at the
bottom (a normal ``fill`` + ``Enter``/click) — and only *after* the
stream starts do they scroll up to read earlier messages. The no-yank
scenarios follow exactly that sequence, so no off-screen input
manipulation is needed (and Playwright's own click/fill auto-scroll
never fires, because the composer is already in view).

Determinism note: the mock paces every SSE frame at 0.02s, so the long
answer streams for several seconds — "mid-stream" assertions land
comfortably inside the window on headless Chromium. Every "held still"
assertion compares against the exact ``scrollTo(0, 0)`` position
(tolerance 5px for rounding).

Test → story mapping (Playwright Mapping Rule):
1. ``test_submit_reveals_new_message``
2. ``test_stream_follows_while_pinned_at_bottom``
3. ``test_no_yank_while_scrolled_up_during_answer_stream``
4. ``test_no_yank_while_scrolled_up_during_thinking``
5. ``test_restore_lands_on_latest_message``
"""
from __future__ import annotations

import asyncio
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 Page, 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"

#: Mirror of app.js's exported ``NEAR_BOTTOM_PX`` (the 200px composer-zone
#: band that counts as "pinned to the bottom").
NEAR_BOTTOM_PX = 200

#: Mock long-answer trigger (~900 words ≈ 8s of streaming at the mock's
#: 0.02s/frame pace) — the wide, deterministic window to scroll away in.
LONG_QUESTION = "write a long answer about my kubernetes cluster"
#: Phase-17 thinking prefix + the long-answer trigger: both mock triggers
#: fire independently (a ~1.3s reasoning stream, then the long answer).
THINK_LONG_QUESTION = "think out loud — write a long answer about my kubernetes cluster"
#: The mock long answer's unique final line (mock_llm.LONG_ANSWER_END) —
#: proves the whole stream landed even while the viewport was at the top.
LONG_ANSWER_END = "LONG-ANSWER-END"
#: Line fragment the mock's deterministic scratchpad carries
#: (mock_llm.compose_thinking) — same key phase 17's suite uses.
THINKING_FRAGMENT = "Step 2: Check my notes"
#: Tolerance for "the viewport held still at the top" (rounding).
HOLD_TOLERANCE_PX = 5


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`` (8 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 == 8
    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 near_bottom(state: dict[str, float]) -> bool:
    """Mirror of app.js's ``isNearBottom`` — the NEAR_BOTTOM_PX band."""
    return state["sh"] - state["y"] - state["ch"] <= NEAR_BOTTOM_PX


def held_at_top(page: Page) -> bool:
    """The viewport has not moved from ``window.scrollTo(0, 0)`` (±5px)."""
    return scroll_state(page)["y"] <= HOLD_TOLERANCE_PX


def wait_settled(page: Page) -> None:
    """The turn is over: the never-stale contract re-enabled the button."""
    expect(page.locator("#send-btn")).to_be_enabled(timeout=30_000)
    expect(page.locator("#send-label")).to_have_text("Send")


def submit(page: Page, question: str) -> None:
    """Submit from the composer — the real-user flow (pinned at the
    bottom, so Playwright's click/fill auto-scroll never kicks in)."""
    page.fill("#message-input", question)
    page.click("#send-btn")
    expect(page.locator(".msg.user .bubble").last).to_contain_text(question)


def brain_bubble_longer_than(n: int) -> str:
    """JS predicate: the LAST brain bubble's rendered text is > n chars
    (i.e. that far into the stream)."""
    return (
        "() => { const els = document.querySelectorAll('.msg.brain .bubble');"
        f" const el = els[els.length - 1]; return !!el && el.innerText.length > {n}; }}"
    )


def brain_message_in_view(page: Page) -> bool:
    """The last brain message intersects the viewport vertically. Partial
    visibility counts: a long answer is taller than the window, and the
    contract is that it is revealed (its lower edge in view), not that it
    fits."""
    box = page.locator(".msg.brain").last.bounding_box()
    if box is None:
        return False
    ch = scroll_state(page)["ch"]
    return box["y"] < ch and box["y"] + box["height"] > 0


# ---------------------------------------------------------------------------
# 1. Submit: the user's message and the answer reveal into view
# ---------------------------------------------------------------------------


def test_submit_reveals_new_message(page: Page, app_url: str, seeded_kb: None) -> None:
    page.set_default_timeout(30_000)
    page.goto(app_url)
    # A fresh chat page starts pinned at the bottom (short conversation —
    # the composer, i.e. the user, sits in the band).
    assert near_bottom(scroll_state(page))

    submit(page, LONG_QUESTION)
    wait_settled(page)

    # The last brain message is inside the viewport ...
    assert brain_message_in_view(page), (
        "the answer must be revealed — the last brain message is not in view"
    )
    # ... and the page is still pinned at the bottom.
    assert near_bottom(scroll_state(page))


# ---------------------------------------------------------------------------
# 2. Follow: while pinned, the page keeps up with the stream
# ---------------------------------------------------------------------------


def test_stream_follows_while_pinned_at_bottom(page: Page, app_url: str, seeded_kb: None) -> None:
    page.set_default_timeout(30_000)
    page.goto(app_url)
    submit(page, LONG_QUESTION)

    # ~2s into the stream: the answer bubble already carries >200 chars
    # (the mock paces frames at 0.02s).
    page.wait_for_function(brain_bubble_longer_than(200), timeout=30_000)
    # Let the smooth follow scroll settle before measuring.
    time.sleep(0.3)
    # The follow behavior is alive — not accidentally removed.
    assert near_bottom(scroll_state(page)), (
        "the page must follow the stream while the user is pinned at the bottom"
    )

    wait_settled(page)
    assert near_bottom(scroll_state(page))


# ---------------------------------------------------------------------------
# 3. No yank: scrolled up mid-ANSWER — the viewport holds for the rest
#    of the turn (the answer finishes off-screen below, by design)
# ---------------------------------------------------------------------------


def test_no_yank_while_scrolled_up_during_answer_stream(
    page: Page, app_url: str, seeded_kb: None
) -> None:
    page.set_default_timeout(30_000)
    page.goto(app_url)

    # Turn 1 (settled) makes the document overflow the 800px viewport.
    submit(page, LONG_QUESTION)
    wait_settled(page)
    state = scroll_state(page)
    assert state["sh"] > state["ch"], "a long answer must make the document scrollable"
    assert near_bottom(state), "follow was active: the settled turn ends pinned"

    # Turn 2: submit from the composer (pinned — normal flow), then let
    # the new answer stream a bit.
    submit(page, LONG_QUESTION)
    page.wait_for_function(brain_bubble_longer_than(200), timeout=30_000)

    # The user goes up to read while the stream is running.
    page.evaluate("() => window.scrollTo(0, 0)")
    # The stream kept running at the top ...
    page.wait_for_function(brain_bubble_longer_than(600), timeout=30_000)
    assert held_at_top(page), "the viewport must hold still while scrolled up"

    # ... and nothing scrolls for the rest of the turn — the answer
    # finishes off-screen below, by design.
    wait_settled(page)
    assert held_at_top(page)


# ---------------------------------------------------------------------------
# 4. No yank: scrolled up during THINKING — the whole reasoning stream
#    plus the answer's start happen at the top
# ---------------------------------------------------------------------------


def test_no_yank_while_scrolled_up_during_thinking(
    page: Page, app_url: str, seeded_kb: None
) -> None:
    page.set_default_timeout(30_000)
    page.goto(app_url)

    # One settled turn first, so the document overflows (scrollable).
    submit(page, LONG_QUESTION)
    wait_settled(page)

    # The thinking turn: submit pinned (normal flow) ...
    submit(page, THINK_LONG_QUESTION)
    details = page.locator(".msg.brain").last.locator("details.thinking")
    details.wait_for(state="attached", timeout=10_000)
    # ... and, while the reasoning stream is still open (phase-17
    # behavior: created open, ~1.3s before the first answer token) ...
    expect(details).to_have_attribute("open", "")
    # ... the user goes up to read.
    page.evaluate("() => window.scrollTo(0, 0)")

    # The whole thinking stream plus the answer's start happen at the top.
    bubble = page.locator(".msg.brain").last.locator(".bubble")
    expect(bubble).not_to_have_text("", timeout=30_000)
    assert held_at_top(page), "the viewport must hold still during thinking"

    # Settled: still at the top, and everything landed (off-screen,
    # which is the point of the story).
    wait_settled(page)
    assert held_at_top(page)
    expect(details.locator(".thinking-text")).to_contain_text(THINKING_FRAGMENT)
    expect(bubble).to_contain_text(LONG_ANSWER_END)


# ---------------------------------------------------------------------------
# 5. Restore: the one-shot landing still puts the latest message in view
#    (phase 14 behavior preserved — pinned so a future "remove all
#    scrolling" change fails loudly instead of silently)
# ---------------------------------------------------------------------------


def test_restore_lands_on_latest_message(page: Page, app_url: str, seeded_kb: None) -> None:
    page.set_default_timeout(30_000)
    page.goto(app_url)

    # Two settled turns (user + brain × 2) — the document overflows.
    submit(page, LONG_QUESTION)
    wait_settled(page)
    submit(page, LONG_QUESTION)
    wait_settled(page)

    page.reload()
    # Restore re-renders from localStorage; wait until the last restored
    # brain answer is fully back.
    expect(page.locator(".msg.brain .bubble").last).to_contain_text(
        LONG_ANSWER_END, timeout=30_000
    )
    wait_settled(page)
    expect(page.locator(".msg.user .bubble")).to_have_count(2)
    state = scroll_state(page)
    assert state["sh"] > state["ch"]

    # The forced one-shot landing (the only `force`d scrolls) puts the
    # last brain message back in view ...
    assert brain_message_in_view(page), (
        "a restored conversation must land on its latest message"
    )
    # ... and the page sits at the bottom.
    assert near_bottom(state)