@samitouri / QOS-React-1 / commits / a96a0f3903

Fix fragmentInstance#compareDocumentPosition nesting and portal cases (#34069)

Found a couple of issues while integrating FragmentInstance#compareDocumentPosition into Fabric. 1. Basic checks of nested host instances were inaccurate. For example, checking the first child of the first child of the Fragment would not return CONTAINED_BY. 2. Then fixing that logic exposed issues with Portals. The DOM positioning relied on the assumption that the first and last top-level children were in the same order as the Fiber tree. I added additional checks against the parent's position in the DOM, and special cased a portaled Fragment by getting its DOM parent from the child instance, rather than taking the instance from the Fiber return. This should be accurate in more cases. Though its still a guess and I'm not sure yet I've covered every variation of this. Portals are hard to deal with and we may end up having to push more results towards IMPLEMENTATION_SPECIFIC if accuracy is an issue.

Jack Pope committed Aug 15, 2025 at 12:14 UTC a96a0f3903ea0a9d45ff7c30a3fd9efe830c4628

3 files changed +219 -44

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

+56 -16

@@ -38,9 +38,16 @@ import hasOwnProperty from 'shared/hasOwnProperty';

38 import {checkAttributeStringCoercion} from 'shared/CheckStringCoercion';

39 import {REACT_CONTEXT_TYPE} from 'shared/ReactSymbols';

40 import {

41 - isFiberContainedBy,

41 + isFiberContainedByFragment,

42 isFiberFollowing,

43 isFiberPreceding,

44 + isFragmentContainedByFiber,

45 + traverseFragmentInstance,

46 + getFragmentParentHostFiber,

47 + getNextSiblingHostFiber,

48 + getInstanceFromHostFiber,

49 + traverseFragmentInstanceDeeply,

50 + fiberIsPortaledIntoHost,

51 } from 'react-reconciler/src/ReactFiberTreeReflection';

53 export {

@@ -63,13 +70,6 @@ import {

70 markNodeAsHoistable,

71 isOwnedInstance,

72 } from './ReactDOMComponentTree';

66 -import {

67 - traverseFragmentInstance,

68 - getFragmentParentHostFiber,

69 - getNextSiblingHostFiber,

70 - getInstanceFromHostFiber,

71 - traverseFragmentInstanceDeeply,

72 -} from 'react-reconciler/src/ReactFiberTreeReflection';

74 export {detachDeletedInstance};

75 import {hasRole} from './DOMAccessibilityRoles';

@@ -3052,13 +3052,13 @@ FragmentInstance.prototype.compareDocumentPosition = function (

3052 }

3053 const children: Array<Fiber> = [];

3054 traverseFragmentInstance(this._fragmentFiber, collectChildren, children);

3055 + const parentHostInstance =

3056 + getInstanceFromHostFiber<Instance>(parentHostFiber);

3057

3058 let result = Node.DOCUMENT_POSITION_DISCONNECTED;

3059 if (children.length === 0) {

3060 // If the fragment has no children, we can use the parent and

3061 // siblings to determine a position.

3060 - const parentHostInstance =

3061 - getInstanceFromHostFiber<Instance>(parentHostFiber);

3062 const parentResult = parentHostInstance.compareDocumentPosition(otherNode);

3063 result = parentResult;

3064 if (parentHostInstance === otherNode) {

@@ -3095,15 +3095,53 @@ FragmentInstance.prototype.compareDocumentPosition = function (

3095 const lastElement = getInstanceFromHostFiber<Instance>(

3096 children[children.length - 1],

3097 );

3098 +

3099 + // If the fragment has been portaled into another host instance, we need to

3100 + // our best guess is to use the parent of the child instance, rather than

3101 + // the fiber tree host parent.

3102 + const parentHostInstanceFromDOM = fiberIsPortaledIntoHost(this._fragmentFiber)

3103 + ? (getInstanceFromHostFiber<Instance>(children[0]).parentElement: ?Instance)

3104 + : parentHostInstance;

3105 +

3106 + if (parentHostInstanceFromDOM == null) {

3107 + return Node.DOCUMENT_POSITION_DISCONNECTED;

3108 + }

3109 +

3110 + // Check if first and last element are actually in the expected document position

3111 + // before relying on them as source of truth for other contained elements

3112 + const firstElementIsContained =

3113 + parentHostInstanceFromDOM.compareDocumentPosition(firstElement) &

3114 + Node.DOCUMENT_POSITION_CONTAINED_BY;

3115 + const lastElementIsContained =

3116 + parentHostInstanceFromDOM.compareDocumentPosition(lastElement) &

3117 + Node.DOCUMENT_POSITION_CONTAINED_BY;

3118 const firstResult = firstElement.compareDocumentPosition(otherNode);

3119 const lastResult = lastElement.compareDocumentPosition(otherNode);

3120 +

3121 + const otherNodeIsFirstOrLastChild =

3122 + (firstElementIsContained && firstElement === otherNode) ||

3123 + (lastElementIsContained && lastElement === otherNode);

3124 + const otherNodeIsFirstOrLastChildDisconnected =

3125 + (!firstElementIsContained && firstElement === otherNode) ||

3126 + (!lastElementIsContained && lastElement === otherNode);

3127 + const otherNodeIsWithinFirstOrLastChild =

3128 + firstResult & Node.DOCUMENT_POSITION_CONTAINED_BY ||

3129 + lastResult & Node.DOCUMENT_POSITION_CONTAINED_BY;

3130 + const otherNodeIsBetweenFirstAndLastChildren =

3131 + firstElementIsContained &&

3132 + lastElementIsContained &&

3133 + firstResult & Node.DOCUMENT_POSITION_FOLLOWING &&

3134 + lastResult & Node.DOCUMENT_POSITION_PRECEDING;

3135 +

3136 if (

3101 - (firstResult & Node.DOCUMENT_POSITION_FOLLOWING &&

3102 - lastResult & Node.DOCUMENT_POSITION_PRECEDING) ||

3103 - otherNode === firstElement ||

3104 - otherNode === lastElement

3137 + otherNodeIsFirstOrLastChild ||

3138 + otherNodeIsWithinFirstOrLastChild ||

3139 + otherNodeIsBetweenFirstAndLastChildren

3140 ) {

3141 result = Node.DOCUMENT_POSITION_CONTAINED_BY;

3142 + } else if (otherNodeIsFirstOrLastChildDisconnected) {

3143 + // otherNode has been portaled into another container

3144 + result = Node.DOCUMENT_POSITION_IMPLEMENTATION_SPECIFIC;

3145 } else {

3146 result = firstResult;

3147 }

@@ -3141,7 +3179,9 @@ function validateDocumentPositionWithFiberTree(

3179 ): boolean {

3180 const otherFiber = getClosestInstanceFromNode(otherNode);

3181 if (documentPosition & Node.DOCUMENT_POSITION_CONTAINED_BY) {

3144 - return !!otherFiber && isFiberContainedBy(fragmentFiber, otherFiber);

3182 + return (

3183 + !!otherFiber && isFiberContainedByFragment(otherFiber, fragmentFiber)

3184 + );

3185 }

3186 if (documentPosition & Node.DOCUMENT_POSITION_CONTAINS) {

3187 if (otherFiber === null) {

@@ -3149,7 +3189,7 @@ function validateDocumentPositionWithFiberTree(

3189 const ownerDocument = otherNode.ownerDocument;

3190 return otherNode === ownerDocument || otherNode === ownerDocument.body;

3191 }

3152 - return isFiberContainedBy(otherFiber, fragmentFiber);

3192 + return isFragmentContainedByFiber(fragmentFiber, otherFiber);

3193 }

3194 if (documentPosition & Node.DOCUMENT_POSITION_PRECEDING) {

3195 return (

packages/react-dom/src/__tests__/ReactDOMFragmentRefs-test.js

+121 -18

@@ -1197,14 +1197,14 @@ describe('FragmentRefs', () => {

1197

1198 function Test() {

1199 return (

1200 - <div ref={containerRef}>

1201 - <div ref={beforeRef} />

1200 + <div ref={containerRef} id="container">

1201 + <div ref={beforeRef} id="before" />

1202 <React.Fragment ref={fragmentRef}>

1203 - <div ref={firstChildRef} />

1204 - <div ref={middleChildRef} />

1205 - <div ref={lastChildRef} />

1203 + <div ref={firstChildRef} id="first" />

1204 + <div ref={middleChildRef} id="middle" />

1205 + <div ref={lastChildRef} id="last" />

1206 </React.Fragment>

1207 - <div ref={afterRef} />

1207 + <div ref={afterRef} id="after" />

1208 </div>

1209 );

1210 }

@@ -1289,7 +1289,7 @@ describe('FragmentRefs', () => {

1289 },

1290 );

1291

1292 - // containerRef preceds and contains the fragment

1292 + // containerRef precedes and contains the fragment

1293 expectPosition(

1294 fragmentRef.current.compareDocumentPosition(containerRef.current),

1295 {

@@ -1328,7 +1328,7 @@ describe('FragmentRefs', () => {

1328 function Test() {

1329 return (

1330 <div id="container" ref={containerRef}>

1331 - <div>

1331 + <div id="innercontainer">

1332 <div ref={beforeRef} id="before" />

1333 <React.Fragment ref={fragmentRef}>

1334 <div ref={onlyChildRef} id="within" />

@@ -1491,6 +1491,77 @@ describe('FragmentRefs', () => {

1491 );

1492 });

1493

1494 + // @gate enableFragmentRefs

1495 + it('handles nested children', async () => {

1496 + const fragmentRef = React.createRef();

1497 + const nestedFragmentRef = React.createRef();

1498 + const childARef = React.createRef();

1499 + const childBRef = React.createRef();

1500 + const childCRef = React.createRef();

1501 + document.body.appendChild(container);

1502 + const root = ReactDOMClient.createRoot(container);

1503 +

1504 + function Child() {

1505 + return (

1506 + <div ref={childCRef} id="C">

1507 + C

1508 + </div>

1509 + );

1510 + }

1511 +

1512 + function Test() {

1513 + return (

1514 + <React.Fragment ref={fragmentRef}>

1515 + <div ref={childARef} id="A">

1516 + A

1517 + </div>

1518 + <React.Fragment ref={nestedFragmentRef}>

1519 + <div ref={childBRef} id="B">

1520 + B

1521 + </div>

1522 + </React.Fragment>

1523 + <Child />

1524 + </React.Fragment>

1525 + );

1526 + }

1527 +

1528 + await act(() => root.render(<Test />));

1529 +

1530 + expectPosition(

1531 + fragmentRef.current.compareDocumentPosition(childARef.current),

1532 + {

1533 + preceding: false,

1534 + following: false,

1535 + contains: false,

1536 + containedBy: true,

1537 + disconnected: false,

1538 + implementationSpecific: false,

1539 + },

1540 + );

1541 + expectPosition(

1542 + fragmentRef.current.compareDocumentPosition(childBRef.current),

1543 + {

1544 + preceding: false,

1545 + following: false,

1546 + contains: false,

1547 + containedBy: true,

1548 + disconnected: false,

1549 + implementationSpecific: false,

1550 + },

1551 + );

1552 + expectPosition(

1553 + fragmentRef.current.compareDocumentPosition(childCRef.current),

1554 + {

1555 + preceding: false,

1556 + following: false,

1557 + contains: false,

1558 + containedBy: true,

1559 + disconnected: false,

1560 + implementationSpecific: false,

1561 + },

1562 + );

1563 + });

1564 +

1565 // @gate enableFragmentRefs

1566 it('returns disconnected for comparison with an unmounted fragment instance', async () => {

1567 const fragmentRef = React.createRef();

@@ -1551,11 +1622,11 @@ describe('FragmentRefs', () => {

1622

1623 function Test() {

1624 return (

1554 - <div>

1555 - {createPortal(<div ref={portaledSiblingRef} />, document.body)}

1625 + <div id="wrapper">

1626 + {createPortal(<div ref={portaledSiblingRef} id="A" />, container)}

1627 <Fragment ref={fragmentRef}>

1557 - {createPortal(<div ref={portaledChildRef} />, document.body)}

1558 - <div />

1628 + {createPortal(<div ref={portaledChildRef} id="B" />, container)}

1629 + <div id="C" />

1630 </Fragment>

1631 </div>

1632 );

@@ -1600,6 +1671,8 @@ describe('FragmentRefs', () => {

1671 const childARef = React.createRef();

1672 const childBRef = React.createRef();

1673 const childCRef = React.createRef();

1674 + const childDRef = React.createRef();

1675 + const childERef = React.createRef();

1676

1677 function Test() {

1678 const [c, setC] = React.useState(false);

@@ -1612,23 +1685,30 @@ describe('FragmentRefs', () => {

1685 {createPortal(

1686 <Fragment ref={fragmentRef}>

1687 <div id="A" ref={childARef} />

1615 - {c ? <div id="C" ref={childCRef} /> : null}

1688 + {c ? (

1689 + <div id="C" ref={childCRef}>

1690 + <div id="D" ref={childDRef} />

1691 + </div>

1692 + ) : null}

1693 </Fragment>,

1694 document.body,

1695 )}

1696 {createPortal(<p id="B" ref={childBRef} />, document.body)}

1697 + <div id="E" ref={childERef} />

1698 </>

1699 );

1700 }

1701

1702 await act(() => root.render(<Test />));

1703

1626 - // Due to effect, order is A->B->C

1627 - expect(document.body.innerHTML).toBe(

1628 - '<div></div>' +

1704 + // Due to effect, order is E / A->B->C->D

1705 + expect(document.body.outerHTML).toBe(

1706 + '<body>' +

1707 + '<div><div id="E"></div></div>' +

1708 '<div id="A"></div>' +

1709 '<p id="B"></p>' +

1631 - '<div id="C"></div>',

1710 + '<div id="C"><div id="D"></div></div>' +

1711 + '</body>',

1712 );

1713

1714 expectPosition(

@@ -1642,7 +1722,6 @@ describe('FragmentRefs', () => {

1722 implementationSpecific: false,

1723 },

1724 );

1645 -

1725 expectPosition(

1726 fragmentRef.current.compareDocumentPosition(childARef.current),

1727 {

@@ -1654,6 +1733,7 @@ describe('FragmentRefs', () => {

1733 implementationSpecific: false,

1734 },

1735 );

1736 + // Contained by in DOM, but following in React tree

1737 expectPosition(

1738 fragmentRef.current.compareDocumentPosition(childBRef.current),

1739 {

@@ -1676,6 +1756,29 @@ describe('FragmentRefs', () => {

1756 implementationSpecific: false,

1757 },

1758 );

1759 + expectPosition(

1760 + fragmentRef.current.compareDocumentPosition(childDRef.current),

1761 + {

1762 + preceding: false,

1763 + following: false,

1764 + contains: false,

1765 + containedBy: true,

1766 + disconnected: false,

1767 + implementationSpecific: false,

1768 + },

1769 + );

1770 + // Preceding DOM but following in React tree

1771 + expectPosition(

1772 + fragmentRef.current.compareDocumentPosition(childERef.current),

1773 + {

1774 + preceding: false,

1775 + following: false,

1776 + contains: false,

1777 + containedBy: false,

1778 + disconnected: false,

1779 + implementationSpecific: true,

1780 + },

1781 + );

1782 });

1783

1784 // @gate enableFragmentRefs

packages/react-reconciler/src/ReactFiberTreeReflection.js

+42 -10

@@ -26,6 +26,7 @@ import {

26 ActivityComponent,

27 SuspenseComponent,

28 OffscreenComponent,

29 + Fragment,

30 } from './ReactWorkTags';

31 import {NoFlags, Placement, Hydrating} from './ReactFiberFlags';

@@ -405,6 +406,21 @@ export function getFragmentParentHostFiber(fiber: Fiber): null | Fiber {

406 return null;

407 }

408

409 +export function fiberIsPortaledIntoHost(fiber: Fiber): boolean {

410 + let foundPortalParent = false;

411 + let parent = fiber.return;

412 + while (parent !== null) {

413 + if (parent.tag === HostPortal) {

414 + foundPortalParent = true;

415 + }

416 + if (parent.tag === HostRoot || parent.tag === HostComponent) {

417 + break;

418 + }

419 + parent = parent.return;

420 + }

421 + return foundPortalParent;

422 +}

423 +

424 export function getInstanceFromHostFiber<I>(fiber: Fiber): I {

425 switch (fiber.tag) {

426 case HostComponent:

@@ -443,22 +459,38 @@ function findNextSibling(child: Fiber): boolean {

459 return true;

460 }

461

446 -export function isFiberContainedBy(

447 - maybeChild: Fiber,

448 - maybeParent: Fiber,

462 +export function isFiberContainedByFragment(

463 + fiber: Fiber,

464 + fragmentFiber: Fiber,

465 ): boolean {

450 - let parent = maybeParent.return;

451 - if (parent === maybeChild || parent === maybeChild.alternate) {

452 - return true;

466 + let current: Fiber | null = fiber;

467 + while (current !== null) {

468 + if (

469 + current.tag === Fragment &&

470 + (current === fragmentFiber || current.alternate === fragmentFiber)

471 + ) {

472 + return true;

473 + }

474 + current = current.return;

475 }

454 - while (parent !== null && parent !== maybeChild) {

476 + return false;

477 +}

478 +

479 +export function isFragmentContainedByFiber(

480 + fragmentFiber: Fiber,

481 + otherFiber: Fiber,

482 +): boolean {

483 + let current: Fiber | null = fragmentFiber;

484 + const fiberHostParent: Fiber | null =

485 + getFragmentParentHostFiber(fragmentFiber);

486 + while (current !== null) {

487 if (

456 - (parent.tag === HostComponent || parent.tag === HostRoot) &&

457 - (parent.return === maybeChild || parent.return === maybeChild.alternate)

488 + (current.tag === HostComponent || current.tag === HostRoot) &&

489 + (current === fiberHostParent || current.alternate === fiberHostParent)

490 ) {

491 return true;

492 }

461 - parent = parent.return;

493 + current = current.return;

494 }

495 return false;

496 }