perf: carry the sibling anchor instead of rescanning per child

[letstri]

renderChildChain finds each child's anchor by scanning every later sibling
through firstDomNode, so mounting a list into a parent that already has
children is O(n²):

<section>
  <span>head</span>
  {rows.map((row) => <Row key={row.id} {...row} />)}   // 2000 of these
</section>

The first DOM node a later sibling owns holds until that sibling renders, so
this caches it instead of rescanning. The cache is kept only while that node is
still a child of this parent and still preceded by what the chain placed there,
so an inline portal move, a nested flushSync reveal, a replaced sibling, or an
anchor moved to another parent alongside its neighbour all drop it and the next
child rescans. Commit-mode insertions are queued, so a child that just mounted
owns no DOM here yet and the node ahead simply has to be where it was.

skipAnchors is gone: the cache reproduces it — a first mount scans once, finds
nothing ahead, and every child uses the parent's anchor — and with correct
anchors on first mounts the follow-up placeChildrenInOrder pass there is dead.

firstDomNode calls, rows appended after a retained header with no trailing
sibling:

New rows	Before	After
200	20,100	200
400	80,200	400
2,000	2,001,000	2,000

Chrome, flushSync mount, medians of 3 runs × 15 samples, interleaved with the
base build, React 19 measured on the same page:

rows	before	after	React 19
2000	11.5ms	5.2ms	9.4ms
8000	106ms	23.5ms	98.9ms

Other workloads from examples/perf-bench, same method: canonical rerender
38.9–42.6ms before vs 41.2–41.7ms after, keyed reorder 16.2–16.7 vs 16.0–17.2,
state churn 10.0–10.1 vs 10.6–11.1 — all within the run-to-run spread.

Size, against this branch's base: dist redact/dom-client 24968 → 24955 B gzip
(−13), client total −4, nano −11, portal=stub −16; dom +9, hydration=stub +18,
fragmentRefs=stub +4, context=stub +3, all within budget.
dist redact/dom-client (viewTransitions=stub) is over its budget at 20160 B,
but it already is on the base commit at 20162 B — this revision is 2 B under it.

Tests: tests/child-chain-cache-regressions.test.tsx and
tests/null-sibling-anchors.test.tsx pass (6/6, plus a new case there for an
anchor moved to another parent with its neighbour), along with the full suite
(1570), test:types and build.

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📝 Walkthrough

Walkthrough

renderChildChain now caches a later sibling’s DOM anchor during child reconciliation. It rescans when the cached node changes, moves, or disconnects. A regression test covers movement to another parent, and the changeset records benchmark results.

Changes

Child Chain Anchor Optimization

Layer / File(s)	Summary
Anchor reconciliation and regression coverage packages/redact/src/dom/reconcile.ts, tests/null-sibling-anchors.test.tsx, .changeset/child-chain-anchors.md	The reconciliation paths now use cached sibling anchors without skip-anchor flags. renderChildChain rescans when the cached anchor is invalid or belongs to another parent. The test covers anchor movement, and the changeset records the patch release and benchmark results.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Refactor

Merge Risk: ⚪ Minimal · up to c901f

No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main performance change: carrying the sibling anchor to avoid rescanning for each child.

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[tannerlinsley]

Checked c85cb147b0d14b5d268461b1dbbc12b5b8c4cd1f on top of the compatibility fixes in #29. There are three things to address before this goes in.

1. The carried anchor can put siblings in the wrong order. Extend the existing inline-portal test to have two empty Middle siblings between First and the portal. When First moves the portal DOM inline, checking only the adjacent sibling misses the new anchor farther ahead. The result is two, one, tail instead of one, two, tail. This fails in both Chrome and jsdom, while the unchanged renderer passes. Regression tests and the candidate test config are in #29.

2. An exhausted scan starts again. When there is no trailing DOM-owning sibling, scanned ends up null, then scanned ?? sibling.sibling restarts the scan on the next child. With a retained header followed by newly mounted rows, instrumenting firstDomNode gives the same counts before and after:

   New rows	Before	This revision
   200	20,100	20,100
   400	80,200	80,200
   2,000	2,001,000	2,001,000

   The fixture is <section><b>head</b>{rows.map(row => <Row key={row.id} {...row} />)}</section>, updated from zero rows, with no trailing sibling. These are call counts, not instrumented timing measurements.

3. The bundle grows. Using the existing production size harness, applying this revision to #29 changes the combined nano/default/full gzip totals from 15,503/23,301/28,125 bytes to 15,555/23,356/28,189 bytes, respectively. That adds 52/55/64 bytes and exceeds the current default/full release budgets. We need to keep the size budgets unchanged.

The new test command, from #29 with this candidate in a separate source tree:

REDACT_TEST_SOURCE=/path/to/pr30/packages/redact/src pnpm exec vitest run --config scripts/child-chain-cache.config.mjs

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Inline comments:
In `@packages/redact/src/dom/reconcile.ts`:
- Line 632: Update the anchor-reuse condition in the reconciliation logic to
also invalidate cached anchors whose parentNode differs from domParent. Preserve
the existing owner, sibling, and detached-node checks so anchors are reused only
when they remain valid children of the current parent.

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