Bug: `FragmentInstance.compareDocumentPosition` alternates between `FOLLOWING` and `IMPLEMENTATION_SPECIFIC` on every other render

提供されたJSXの再現コードをReact 19.3.0で実行し、再レンダー間で位置マスクが交互になることを確認してください。次に packages/react-reconciler/src/ReactFiberTreeReflection.js を調査し、特に validateDocumentPositionWithFiberTree、isFiberPrecedingCheck、isFiberFollowingCheck を確認してください。再レンダー間で FOLLOWING と PRECEDING の結果が安定し、文書化されている空のケースやポータルのケースを変更しなければ完了です。

[!NOTE]

AI Disclosure: An AI (Claude Fable 5.1) wrote the issue below. I checked it and take responsibility for it.

React version: 19.3.0 (react and react-dom). The relevant validation code
looks unchanged on current main, so I believe it is still affected.

Steps To Reproduce

1. Render a <Fragment ref={fragmentRef}> with at least one DOM child, plus a
   sibling element after the fragment under the same parent.
2. Call fragmentRef.current.compareDocumentPosition(afterSibling) and then
   trigger any state update (re-render).
3. Repeat step 2.

import { Fragment, useRef, useState } from 'react';

function App() {
  const fragmentRef = useRef(null);
  const [results, setResults] = useState([]);
  return (
    <div>
      <button
        onClick={() => {
          const mask = fragmentRef.current.compareDocumentPosition(
            document.getElementById('after'),
          );
          // The state update re-renders, which flips the result of the
          // *next* click.
          setResults((prev) => [...prev, mask]);
        }}
      >
        compareDocumentPosition(afterSibling)
      </button>
      <Fragment ref={fragmentRef}>
        <p>fragment child</p>
      </Fragment>
      <p id="after">sibling after the fragment</p>
      <pre>{results.join(', ')}</pre>
    </div>
  );
}

Link to code example: https://github.com/uhyo/react-193-playground/blob/51307a38a34202644611223a3f73e415b5ae1640/docs/repro-compare-document-position.html

The current behavior

The result alternates with render parity:

4, 32, 4, 32, 4, 32

i.e. DOCUMENT_POSITION_FOLLOWING on even render generations and bare
DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC on odd ones. The
PRECEDING case (a sibling before the fragment) is affected the same way on
odd generations. Queries for nodes inside the fragment (CONTAINED_BY) are
stable.

Reproduced in Chromium with react-dom 19.3.0, in both development and
production builds, with and without StrictMode.

The expected behavior

A stable DOCUMENT_POSITION_FOLLOWING (4) / DOCUMENT_POSITION_PRECEDING (2)
regardless of how many times the tree has re-rendered. Per the
Fragment docs,
DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC is expected for empty
Fragments and Fragments with children rendered through a portal — neither
applies here.