hermes-webui/tests/test_issue4295_midstream_scroll_anchor.py

"""Regression for #4295: keep mid-stream reader anchor while streaming.

The fix needs a scroll restore that keeps the semantic message anchor authoritative
even when DOM height grows while the user is manually unpinned.
"""

import subprocess
from pathlib import Path

REPO = Path(__file__).resolve().parents[1]
UI_JS = (REPO / "static" / "ui.js").read_text(encoding="utf-8")


def _function_body(src: str, name: str) -> str:
    marker = f"function {name}"
    start = src.find(marker)
    assert start >= 0, f"{name} not found"
    brace = src.find("{", start)
    assert brace >= 0, f"{name} body not found"
    depth = 0
    for i, ch in enumerate(src[brace:], brace):
        if ch == "{":
            depth += 1
        elif ch == "}":
            depth -= 1
            if depth == 0:
                return src[start : i + 1]
    raise AssertionError(f"{name} body did not terminate")


def _compact(text: str) -> str:
    return "".join(text.split())


def test_restore_message_scroll_snapshot_remounts_virtual_anchor_and_keeps_unpinned():
    """Drive the real restore helpers with a tiny DOM stub.

    This covers the #4295 failure mode where a mid-stream render rebuild has to
    remount a virtualized anchor before restoring the semantic reader position.
    The geometry is intentionally near-bottom after restore; old distance-based
    state inference would incorrectly re-pin the reader.
    """

    script = f"""
const assert = require('assert');
let _programmaticScroll = false;
let _messageVirtualWindowKey = 'old-window';
let _lastScrollTop = 0;
let _messageUserUnpinned = false;
let _scrollPinned = true;
let _nearBottomCount = 2;
let _messageViewportAnchorRemounting = false;
let targetMounted = false;
const renderCalls = [];
const targetRow = {{
  getBoundingClientRect() {{ return {{ top: 140, bottom: 180 }}; }}
}};
const container = {{
  scrollTop: 300,
  scrollHeight: 905,
  clientHeight: 400,
  getBoundingClientRect() {{ return {{ top: 100, bottom: 500 }}; }},
  querySelector(selector) {{
    if (selector === '[data-msg-idx="20"]' && targetMounted) return targetRow;
    return null;
  }}
}};
function $(id) {{ return id === 'messages' ? container : null; }}
function requestAnimationFrame(fn) {{ fn(); }}
function setTimeout(fn) {{ fn(); }}
function _getVisibleMessagesWithIdx() {{
  return [{{ rawIdx: 5 }}, {{ rawIdx: 20 }}, {{ rawIdx: 42 }}];
}}
function _messageVisibleIndexForRawIdx(rawIdx, visWithIdx) {{
  return visWithIdx.findIndex((entry) => entry && entry.rawIdx === rawIdx);
}}
function _messageVirtualScrollTopForVisibleIdx(visWithIdx, visIdx, el) {{
  assert.strictEqual(visIdx, 1);
  assert.strictEqual(el, container);
  return 480;
}}
function renderMessages(options) {{
  renderCalls.push(options);
  assert.strictEqual(_messageViewportAnchorRemounting, true);
  targetMounted = true;
}}
{_function_body(UI_JS, "_restoreMessageViewportAnchor")}
{_function_body(UI_JS, "_remountMessageViewportAnchor")}
{_function_body(UI_JS, "_restorePinnedMessageScrollSnapshot")}
{_function_body(UI_JS, "_restoreMessageScrollSnapshot")}
_restoreMessageScrollSnapshot({{
  anchor: {{ rawIdx: 20, topOffset: 15 }},
  top: 222,
  bottom: 600,
  pinned: false,
  userUnpinned: true,
}});
assert.strictEqual(renderCalls.length, 1);
assert.deepStrictEqual(renderCalls[0], {{ preserveScroll: true }});
assert.strictEqual(_messageVirtualWindowKey, '');
assert.strictEqual(container.scrollTop, 505);
assert.strictEqual(_lastScrollTop, 505);
assert.strictEqual(_messageUserUnpinned, true);
assert.strictEqual(_scrollPinned, false);
assert.strictEqual(_nearBottomCount, 0);
assert.strictEqual(_programmaticScroll, false);
"""
    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)


def test_live_anchor_rebuild_guard_holds_height_and_marks_reader_unpinned():
    """Live anchor rebuilds must not let the scroll container collapse mid-read."""

    script = f"""
const assert = require('assert');
let _messageUserUnpinned = true;
let _scrollPinned = false;
let _nearBottomCount = 2;
const messages = {{
  scrollTop: 2000,
  scrollHeight: 5000,
  clientHeight: 600,
}};
const inner = {{ style: {{ minHeight: '' }}, dataset: {{}} }};
function $(id) {{
  if (id === 'messages') return messages;
  if (id === 'msgInner') return inner;
  return null;
}}
{_function_body(UI_JS, "_prepareLiveAnchorScrollRebuildGuard")}
const snapshot = {{
  top: 2000,
  bottom: 0,
  scrollHeight: 5000,
  pinned: true,
  userUnpinned: false,
}};
const guard = _prepareLiveAnchorScrollRebuildGuard(snapshot);
assert.strictEqual(guard.readerAwayFromBottom, true);
assert.strictEqual(snapshot.pinned, false);
assert.strictEqual(snapshot.userUnpinned, true);
assert.strictEqual(snapshot.bottom, 2400);
assert.strictEqual(_messageUserUnpinned, true);
assert.strictEqual(_scrollPinned, false);
assert.strictEqual(_nearBottomCount, 0);
assert.strictEqual(inner.style.minHeight, '5000px');
assert.strictEqual(inner.dataset.liveAnchorScrollGuardPreviousMinHeight, '');
messages.scrollHeight = 5200;
const nestedSnapshot = {{
  top: 2000,
  bottom: 0,
  scrollHeight: 5200,
  pinned: true,
  userUnpinned: false,
}};
const nestedGuard = _prepareLiveAnchorScrollRebuildGuard(nestedSnapshot);
assert.strictEqual(nestedGuard.readerAwayFromBottom, true);
assert.strictEqual(inner.style.minHeight, '5200px');
assert.strictEqual(inner.dataset.liveAnchorScrollGuardPreviousMinHeight, '');
guard.release();
assert.strictEqual(inner.style.minHeight, '');
nestedGuard.release();
assert.strictEqual(inner.style.minHeight, '');
assert.strictEqual(Object.prototype.hasOwnProperty.call(inner.dataset, 'liveAnchorScrollGuardPreviousMinHeight'), false);
"""
    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)


def test_live_anchor_rebuild_guard_keeps_pinned_follower_pinned_on_large_growth():
    """A PINNED follower must NOT be reclassified as unpinned when a large live
    render transiently pushes bottomDistance>250. Regression guard: the predicate
    must require an explicit non-follow signal (_messageUserUnpinned / _scrollPinned
    === false), not a raw scrollTop>0, or pinned live streams stop auto-following."""

    script = f"""
const assert = require('assert');
let _messageUserUnpinned = false;
let _scrollPinned = true;
let _nearBottomCount = 2;
const messages = {{
  scrollTop: 4400,
  scrollHeight: 5500,
  clientHeight: 600,
}};
const inner = {{ style: {{ minHeight: '' }}, dataset: {{}} }};
function $(id) {{
  if (id === 'messages') return messages;
  if (id === 'msgInner') return inner;
  return null;
}}
{_function_body(UI_JS, "_prepareLiveAnchorScrollRebuildGuard")}
const snapshot = {{
  top: 4400,
  bottom: 0,
  scrollHeight: 5500,
  pinned: true,
  userUnpinned: false,
}};
const guard = _prepareLiveAnchorScrollRebuildGuard(snapshot);
// bottomDistance = 5500 - 4400 - 600 = 500 (>250), but the reader is pinned and
// has not manually unpinned -> must stay pinned, follow-scroll must NOT be skipped.
assert.strictEqual(guard.readerAwayFromBottom, false);
assert.strictEqual(guard.release, null);
assert.strictEqual(_messageUserUnpinned, false);
assert.strictEqual(_scrollPinned, true);
assert.strictEqual(snapshot.pinned, true);
assert.strictEqual(snapshot.userUnpinned, false);
assert.strictEqual(inner.style.minHeight, '');
"""
    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)


def test_restore_message_scroll_snapshot_keeps_intermediate_distance_state():
    script = f"""
const assert = require('assert');
let _programmaticScroll = false;
let _messageVirtualWindowKey = 'old-window';
let _lastScrollTop = 0;
let _messageUserUnpinned = true;
let _scrollPinned = false;
let _nearBottomCount = 0;
let _messageViewportAnchorRemounting = false;
const container = {{
  scrollTop: 10,
  scrollHeight: 800,
  clientHeight: 400,
  getBoundingClientRect() {{ return {{ top: 100, bottom: 500 }}; }},
  querySelector() {{ return null; }}
}};
function $(id) {{ return id === 'messages' ? container : null; }}
function requestAnimationFrame(fn) {{ fn(); }}
function setTimeout(fn) {{ fn(); }}
function _getVisibleMessagesWithIdx() {{ return []; }}
function _messageVisibleIndexForRawIdx() {{ return -1; }}
function _messageVirtualScrollTopForVisibleIdx() {{ throw new Error('not expected'); }}
function renderMessages() {{ throw new Error('not expected'); }}
{_function_body(UI_JS, "_restoreMessageViewportAnchor")}
{_function_body(UI_JS, "_remountMessageViewportAnchor")}
{_function_body(UI_JS, "_restorePinnedMessageScrollSnapshot")}
{_function_body(UI_JS, "_restoreMessageScrollSnapshot")}
_restoreMessageScrollSnapshot({{
  anchor: null,
  top: 200,
  bottom: 200,
  pinned: false,
  userUnpinned: false,
}});
assert.strictEqual(container.scrollTop, 200);
assert.strictEqual(_lastScrollTop, 200);
assert.strictEqual(_messageUserUnpinned, true);
assert.strictEqual(_scrollPinned, false);
assert.strictEqual(_nearBottomCount, 0);
assert.strictEqual(_programmaticScroll, false);
"""
    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)


def test_capture_snapshot_treats_recent_scroll_away_as_manual_reader():
    """Manual live-Worklog browsing must not reuse a stale pinned snapshot.

    A reader can be far from the live tail while scrolling down through a long
    Compact Worklog. If a live activity rebuild captures that state while the
    sticky globals still say pinned, restoring as a pinned follower preserves a
    huge bottom gap and can keep yanking the viewport back toward the turn start.
    Recent message-pane scroll intent makes the snapshot explicitly unpinned.
    """

    script = f"""
const assert = require('assert');
let _messageUserUnpinned = false;
let _scrollPinned = true;
let _lastMessageScrollIntentMs = 1000;
const MESSAGE_WHEEL_INTENT_SUPPRESS_MS = 1200;
const container = {{
  scrollTop: 900,
  scrollHeight: 5000,
  clientHeight: 600,
  getBoundingClientRect() {{ return {{ top: 0, bottom: 600 }}; }},
  querySelectorAll() {{ return []; }},
}};
function $(id) {{ return id === 'messages' ? container : null; }}
const performance = {{ now() {{ return 1500; }} }};
function _captureMessageViewportAnchor() {{ return null; }}
function _recentMessageTouchScrollIntent() {{ return false; }}
function _recentMessageKeyScrollIntent() {{ return false; }}
function _shouldFollowMessagesOnDomReplace() {{ return true; }}
{_function_body(UI_JS, "_recentMessageScrollIntent")}
{_function_body(UI_JS, "_captureMessageScrollSnapshot")}
const snapshot = _captureMessageScrollSnapshot();
assert.strictEqual(snapshot.bottom, 3500);
assert.strictEqual(snapshot.pinned, false);
assert.strictEqual(snapshot.userUnpinned, true);
"""
    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)


def test_capture_snapshot_keeps_true_pinned_follower_pinned_despite_large_gap():
    """Large bottom distance alone is not enough to unpin a follower.

    During fast streaming, content can grow under a followed viewport before the
    follow write lands. Without a recent user scroll intent or explicit unpin
    state, the snapshot must stay pinned so the existing tail-relative restore
    continues to protect pinned live followers. Raw touch/key recency is not
    enough here because those helpers also track near-tail artifact-suppression
    windows; only the guarded away-from-bottom intent stamp can classify a
    snapshot as manual-reader state.
    """

    script = f"""
const assert = require('assert');
let _messageUserUnpinned = false;
let _scrollPinned = true;
let _lastMessageScrollIntentMs = -Infinity;
const MESSAGE_WHEEL_INTENT_SUPPRESS_MS = 1200;
const container = {{
  scrollTop: 900,
  scrollHeight: 5000,
  clientHeight: 600,
  getBoundingClientRect() {{ return {{ top: 0, bottom: 600 }}; }},
  querySelectorAll() {{ return []; }},
}};
function $(id) {{ return id === 'messages' ? container : null; }}
const performance = {{ now() {{ return 1500; }} }};
function _captureMessageViewportAnchor() {{ return null; }}
function _recentMessageTouchScrollIntent() {{ return true; }}
function _recentMessageKeyScrollIntent() {{ return true; }}
function _shouldFollowMessagesOnDomReplace() {{ return true; }}
{_function_body(UI_JS, "_recentMessageScrollIntent")}
{_function_body(UI_JS, "_captureMessageScrollSnapshot")}
const snapshot = _captureMessageScrollSnapshot();
assert.strictEqual(snapshot.bottom, 3500);
assert.strictEqual(snapshot.pinned, true);
assert.strictEqual(snapshot.userUnpinned, false);
"""
    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)


def test_wheel_scroll_intent_only_records_when_reader_is_away_from_bottom():
    """Downward wheel intent should protect manual reading, not bottom following."""

    script = f"""
const assert = require('assert');
let _lastNonMessageScrollIntentMs = -Infinity;
let _lastMessageWheelIntentMs = -Infinity;
let _lastMessageScrollIntentMs = -Infinity;
let _messageUserUnpinned = false;
let _nearBottomCount = 2;
let _scrollPinned = true;
let _messageTouchScrollActive = false;
let _lastMessageTouchScrollIntentMs = -Infinity;
let _touchStartY = null;
const child = {{}};
const el = {{
  scrollTop: 900,
  scrollHeight: 5000,
  clientHeight: 600,
  contains(target) {{ return target === child; }},
}};
function _cancelBottomSettle() {{}}
function _markMessageTouchScrollIntent(active) {{
  _messageTouchScrollActive = !!active;
  _lastMessageTouchScrollIntentMs = performance.now();
}}
const document = {{ getElementById(id) {{ return id === 'messages' ? el : null; }} }};
const performance = {{ now() {{ return 1234; }} }};
{_function_body(UI_JS, "_recordNonMessageScrollIntent")}
_recordNonMessageScrollIntent({{ target: child, type: 'wheel', deltaY: 24 }});
assert.strictEqual(_lastMessageScrollIntentMs, 1234);
assert.strictEqual(_messageUserUnpinned, false);

_lastMessageScrollIntentMs = -Infinity;
el.scrollTop = 4400; // bottomDistance = 0
_recordNonMessageScrollIntent({{ target: child, type: 'wheel', deltaY: 24 }});
assert.strictEqual(_lastMessageScrollIntentMs, -Infinity);
assert.strictEqual(_scrollPinned, true);
"""
    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)


def test_manual_scroll_snapshot_intent_excludes_raw_touch_and_key_recency():
    """Only bottom-guarded message scroll intent can drive snapshot unpinning."""

    compact = _compact(_function_body(UI_JS, "_recentMessageScrollIntent"))

    assert "_lastMessageScrollIntentMs" in compact
    assert "_scrollbarDragActive" in compact
    assert "_recentMessageTouchScrollIntent" not in compact
    assert "_recentMessageKeyScrollIntent" not in compact
    assert "_lastMessageKeyScrollIntentMs=now;" in _compact(UI_JS)
    assert "if(bottomDistance>120)_lastMessageScrollIntentMs=now;" in _compact(UI_JS)


def test_touch_scroll_intent_only_records_when_reader_is_away_from_bottom():
    """Touch recency is broad, but snapshot intent is bottom-distance guarded."""

    script = f"""
const assert = require('assert');
let _lastNonMessageScrollIntentMs = -Infinity;
let _lastMessageWheelIntentMs = -Infinity;
let _lastMessageScrollIntentMs = -Infinity;
let _messageUserUnpinned = false;
let _nearBottomCount = 2;
let _scrollPinned = true;
let _messageTouchScrollActive = false;
let _lastMessageTouchScrollIntentMs = -Infinity;
let _touchStartY = null;
const child = {{}};
const el = {{
  scrollTop: 900,
  scrollHeight: 5000,
  clientHeight: 600,
  contains(target) {{ return target === child; }},
}};
function _cancelBottomSettle() {{}}
function _markMessageTouchScrollIntent(active) {{
  _messageTouchScrollActive = !!active;
  _lastMessageTouchScrollIntentMs = performance.now();
}}
const document = {{ getElementById(id) {{ return id === 'messages' ? el : null; }} }};
const performance = {{ now() {{ return 1234; }} }};
{_function_body(UI_JS, "_recordNonMessageScrollIntent")}
_recordNonMessageScrollIntent({{ target: child, type: 'touchmove' }});
assert.strictEqual(_lastMessageScrollIntentMs, 1234);
assert.strictEqual(_lastMessageTouchScrollIntentMs, 1234);
assert.strictEqual(_messageUserUnpinned, false);

_lastMessageScrollIntentMs = -Infinity;
el.scrollTop = 4400; // bottomDistance = 0
_recordNonMessageScrollIntent({{ target: child, type: 'touchmove' }});
assert.strictEqual(_lastMessageScrollIntentMs, -Infinity);
assert.strictEqual(_lastMessageTouchScrollIntentMs, 1234);
assert.strictEqual(_scrollPinned, true);
"""
    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)


def test_restore_message_scroll_snapshot_prefers_semantic_anchor_with_virtual_fallback():
    body = _function_body(UI_JS, "_restoreMessageScrollSnapshot")
    helper = _function_body(UI_JS, "_remountMessageViewportAnchor")
    compact = _compact(body)
    helper_compact = _compact(helper)

    assert "snapshot.anchor&&typeof_restoreMessageViewportAnchor==='function'" in compact
    assert "if(!restoredViaAnchor&&typeof_remountMessageViewportAnchor==='function'&&_remountMessageViewportAnchor(snapshot.anchor))" in compact
    assert "typeof_getVisibleMessagesWithIdx!=='function'" in helper_compact
    assert "_getVisibleMessagesWithIdx()" in helper_compact
    assert "_messageVisibleIndexForRawIdx(targetIdx,visWithIdx)" in helper_compact
    assert "_messageVirtualScrollTopForVisibleIdx(visWithIdx,visIdx,container)" in helper_compact
    assert "_messageVirtualWindowKey=''" in helper_compact
    assert "renderMessages({preserveScroll:true});" in helper_compact
    assert "_messageViewportAnchorRemounting=true" in helper_compact
    assert "setTimeout(()=>{_programmaticScroll=false;},0);" in helper_compact


def test_restore_message_scroll_snapshot_keeps_user_unpinned_state_authoritative_mid_stream():
    body = _function_body(UI_JS, "_restoreMessageScrollSnapshot")
    compact = _compact(body)

    assert "constbottomDistance=el.scrollHeight-el.scrollTop-el.clientHeight;" in compact
    assert "if(snapshot.userUnpinned===true){_messageUserUnpinned=true;_scrollPinned=false;_nearBottomCount=0;}elseif(snapshot.pinned===true){_messageUserUnpinned=false;_scrollPinned=true;_nearBottomCount=2;}else{" in compact
    assert "_messageUserUnpinned=false;_scrollPinned=false;_nearBottomCount=0;" not in compact


def test_same_frame_restore_uses_the_shared_anchor_remount_fallback():
    body = _function_body(UI_JS, "_restoreMessageScrollSnapshotSameFrame")
    compact = _compact(body)

    assert "if(!restoredViaAnchor&&typeof_remountMessageViewportAnchor==='function'&&_remountMessageViewportAnchor(snapshot.anchor))" in compact
    assert "_messageVirtualScrollTopForVisibleIdx" not in body


def test_stale_content_key_recovers_via_session_index_and_compensates_height():
    """Desktop scroll jump-back root fix (residual, distinct from #5392 mobile path).

    The viewport anchor key is content-derived (role|ts|attachments|first-160-chars),
    so while a live assistant message is streaming, each chunk that changes the first
    160 chars recomputes that row's data-message-anchor-key. A scroll snapshot captured
    a chunk earlier now has a STALE key that no longer matches by key lookup.

    The OLD code conceded immediately on a stale key (`if(!row&&anchorKey) return false`),
    and the caller then fell back to an ABSOLUTE scrollTop that does not compensate the
    above-viewport height change from that same chunk -> desktop jump-back. The row is
    still in the DOM under its stable data-session-msg-idx, so the fix recovers it via
    sessionIdx and applies the relative-offset realign (which compensates the height).

    Behavior test (not a source-string check): the anchor row's on-screen top must be
    held at its captured topOffset. With buggy code the keyed lookup misses, sessionIdx
    is never consulted, _restoreMessageViewportAnchor returns false, and this asserts FAIL.
    """

    script = f"""
const assert = require('assert');
let _programmaticScroll = false;
let _programmaticScrollSetAt = 0;
// Scenario: reader is unpinned mid-transcript. A streaming chunk (a) changed the
// anchor row's content-derived key (stale key -> keyed lookup misses) and (b) grew
// content above the viewport. We assert the realign still runs (via sessionIdx) and
// writes the exact relative-offset compensation instead of conceding to an absolute
// jump. Geometry is chosen so the write is unambiguous:
//   containerRect.top = 100, captured topOffset = 300 (row was 300px below container top)
//   after the chunk, the row's live rect.top = 250 => (250-100) = 150 from container top
//   realign write: scrollTop += (150) - 300 = -150  => 1200 - 150 = 1050
// A buggy body dead-ends on the stale key (returns false, no scrollTop write at all),
// so both asserts below FAIL on the old code.
const anchorRow = {{
  getBoundingClientRect() {{ return {{ top: 250, bottom: 400 }}; }}
}};
const container = {{
  scrollTop: 1200,
  scrollHeight: 4600,
  clientHeight: 600,
  getBoundingClientRect() {{ return {{ top: 100, bottom: 700 }}; }},
  querySelectorAll(selector) {{
    // Stale key: NO node matches the captured (old) content key.
    if (selector === '[data-message-anchor-key]') return [];
    return [];
  }},
  querySelector(selector) {{
    // The row is still present under its stable session index (and raw index).
    if (selector === '[data-session-msg-idx="3"]') return anchorRow;
    if (selector === '[data-msg-idx="3"]') return anchorRow;
    return null;
  }}
}};
function $(id) {{ return id === 'messages' ? container : null; }}
function requestAnimationFrame(fn) {{ fn(); }}
function setTimeout(fn) {{ fn(); }}
const performance = {{ now() {{ return 0; }} }};
function _suppressBrowserOverflowAnchor() {{ return null; }}  // desktop no-op
function _deferClearProgrammaticScroll() {{ _programmaticScroll = false; }}
{_function_body(UI_JS, "_restoreMessageViewportAnchor")}
// Captured snapshot: anchor row was at topOffset=300, content key now STALE, but the
// row still exists at sessionIdx=3 / rawIdx=3.
const ok = _restoreMessageViewportAnchor({{
  key: 'assistant|200|0|STALE_first_160_chars_from_an_earlier_chunk',
  sessionIdx: 3,
  rawIdx: 3,
  topOffset: 300,
}}, 0);
// Must NOT concede on the stale key: sessionIdx recovers the row and the realign runs.
assert.strictEqual(ok, true, 'stale content key must recover via sessionIdx, not return false');
// Relative realign: scrollTop += (rect.top - containerRect.top) - targetTop
//                              = 1200 + ((250 - 100) - 300) = 1200 + (150 - 300) = 1050.
// This compensates the above-viewport height change; the buggy path would have left
// an absolute jump (or no write at all), so this exact value pins the correct behavior.
assert.strictEqual(container.scrollTop, 1050, 'realign must compensate the above-height change, not leave an absolute jump');
"""
    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)


def test_stale_key_does_not_dead_end_before_session_index_fallback():
    """Guard the exact dead-code regression: a present content key must not short-circuit
    the sessionIdx fallback. The old body had `if(!row&&anchorKey) return false;` BEFORE
    sessionIdx was consulted, so a stale key could never reach the session-index recovery.
    This asserts that early dead-end is gone and sessionIdx is resolved before conceding.
    """

    compact = _compact(_function_body(UI_JS, "_restoreMessageViewportAnchor"))

    # The old dead-end (concede on stale key before trying sessionIdx) must be gone.
    assert "if(!row&&anchorKey)returnfalse;" not in compact
    # sessionIdx must be computed and the row recovered by it before any concede.
    assert 'container.querySelector(`[data-session-msg-idx="${{sessionIdx}}"]`)'.replace("{{", "{").replace("}}", "}") in compact
    # The concede now requires BOTH key AND sessionIdx to have failed.
    assert "if(!row&&(anchorKey||hasSessionIdx))returnfalse;" in compact