"""Unit: the sticky-header contract (phase 60, TODO L3).

The browser behavior itself is E2E-gated by the phase-60 story suite
(task 02); like the other frontend-adjacent unit files, this module
pins the CSS markers the sticky contract depends on, so a silent
regression is caught without a browser:

* the ROOT CAUSE (owner-locked A1) is the ``body { height: 100% }``
  cap — a sticky element's travel range is constrained to its
  containing block, and the fixed body height pinned the box to one
  viewport, so ``.app-header`` / ``.doc-header`` un-pinned after
  ~1 viewport of scroll. The FIX (owner-locked A2) is CSS-only:
  ``html`` keeps ``height: 100%`` (harmless viewport baseline) and
  ``body`` carries NO ``height:`` declaration — its existing
  ``min-height: 100dvh`` keeps driving the short-page stretch (the
  phase-52 flex-stretch / pinned-composer / footer contract);
* ``.app-header`` (every page) and ``.doc-header`` (document viewer)
  keep ``position: sticky; top: 0`` — these pins guard against a
  future "simplification" that would drop the sticky rule that was
  always the intent.
"""
from __future__ import annotations

import re
from pathlib import Path

FRONTEND = Path(__file__).resolve().parents[2] / "frontend"
STYLES_CSS = FRONTEND / "assets" / "styles.css"


def _css() -> str:
    return STYLES_CSS.read_text(encoding="utf-8")


def _block(css: str, selector: str) -> str:
    """The declaration body of the FIRST top-level ``<selector> { … }``
    rule. Line-anchored on purpose: the phase-60 provenance comment
    quotes ``body { height: 100% }`` literally, so a mid-line match
    would capture the comment instead of the rule (real rules start at
    column 0)."""
    m = re.search(r"(?m)^" + re.escape(selector) + r" \{([\s\S]*?)\n\}", css)
    assert m, f"styles.css must style {selector}"
    return m.group(1)


# ---------- the root-cause cap is gone (A1 → A2) ----------


def test_body_height_cap_is_gone_from_the_rule() -> None:
    """Phase 60 (A2): the exact old rule ``html, body { height: 100%;
    }`` is GONE from styles.css, and only ``html`` carries
    ``height: 100%`` now (the harmless viewport baseline stays on the
    canvas element)."""
    css = _css()
    assert "html, body { height: 100%; }" not in css, (
        "the combined html,body height rule must be gone"
    )
    assert "html { height: 100%; }" in css, ("html keeps height: 100%")


def test_html_rule_carries_the_phase_60_provenance_comment() -> None:
    """The replacement comment cites the phase 60 provenance (owner
    confirmation 2026-08-31, TODO L3) and names the mechanism — the
    sticky travel range is capped by the containing block, and
    min-height: 100dvh is what stretches short pages."""
    css = _css()
    i = css.find("html { height: 100%; }")
    assert i != -1, "the html height rule must exist"
    comment = css[max(0, i - 900) : i]
    assert "Phase 60" in comment, "the comment cites the phase 60 provenance"
    assert "2026-08-31" in comment, "the comment cites the owner confirmation date"
    assert "TODO L3" in comment, "the comment cites the TODO line"
    assert "min-height: 100dvh" in comment, (
        "the comment names the short-page stretch driver"
    )


def test_body_rule_has_no_height_but_keeps_min_height() -> None:
    """The ``body { … }`` rule: NO ``height:`` declaration (the A1 cap
    must never come back) and ``min-height: 100dvh`` intact — the
    flex-column stretch driver the phase-52 short-page layout (footer
    at the viewport bottom, pinned composer) depends on. The
    declaration list is otherwise UNCHANGED (the flex column
    properties stay)."""
    css = _css()
    body = _block(css, "body")
    assert not re.search(r"(?m)^\s*height\s*:", body), (
        "the body rule must carry NO height declaration"
    )
    assert "min-height: 100dvh" in body, ("the stretch driver stays on body")
    for prop in (
        "margin: 0",
        "display: flex",
        "flex-direction: column",
        "position: relative",
        "background: transparent",
    ):
        assert prop in body, f"the body rule keeps its existing {prop}"


# ---------- the sticky rules stay (both surfaces) ----------


def test_app_header_stays_sticky_at_the_top() -> None:
    """``.app-header`` (the navbar on every page) keeps
    ``position: sticky; top: 0`` (z-index 20, the 64px --header-h
    height) and the phase-12 ``flex-shrink: 0`` guard (reworded phase
    60: body stretches via min-height: 100dvh; the guard still covers
    content-overflow pages, e.g. Sources at ≤640px)."""
    css = _css()
    header = _block(css, ".app-header")
    assert "position: sticky" in header, ".app-header must stay sticky"
    assert "top: 0" in header, ".app-header must pin to the top"
    assert "z-index: 20" in header
    assert "height: var(--header-h)" in header
    assert "flex-shrink: 0" in header, "the shrink guard stays"


def test_doc_header_stays_sticky_at_the_top() -> None:
    """``.doc-header`` (the document viewer's two-row header) keeps
    ``position: sticky; top: 0`` (z-index 20) and the shrink guard —
    the same contract as the app header, so BOTH rows stay pinned
    while the document scrolls."""
    css = _css()
    header = _block(css, ".doc-header")
    assert "position: sticky" in header, ".doc-header must stay sticky"
    assert "top: 0" in header, ".doc-header must pin to the top"
    assert "z-index: 20" in header
    assert "flex-shrink: 0" in header, "the shrink guard stays"


def test_sticky_pin_comment_mentions_the_stretch_driver() -> None:
    """The reworded phase-12 comment on ``.app-header`` (inside the
    rule, above the guard) matches reality: it names
    ``min-height: 100dvh`` as the body stretch driver (the
    "definite-height" wording of the old cap era is gone) and keeps
    the content-overflow rationale for the guard."""
    css = _css()
    comment = _block(css, ".app-header")
    assert "definite-height" not in comment, (
        "the stale definite-height wording must be gone"
    )
    assert "min-height: 100dvh" in comment, (
        "the reworded comment names the actual stretch driver"
    )
    assert "flex-shrink" in comment and "Sources" in comment, (
        "the guard's rationale (content-overflow pages) stays"
    )