diff --git a/tools/build-scripts/packages/codemod-react-legacy-element.mjs b/tools/build-scripts/packages/codemod-react-legacy-element.mjs
index 9b43e0b1733491..1e100a45656566 100644
--- a/tools/build-scripts/packages/codemod-react-legacy-element.mjs
+++ b/tools/build-scripts/packages/codemod-react-legacy-element.mjs
@@ -174,16 +174,125 @@ function patchReference( refPath, legacyName, ms, filename ) {
 }
 
 /**
- * Applies the legacy-element codemod to a bundled React script.
+ * Whether a node is a `<object>.props.ref` member expression.
  *
- * @param {string} code     Source code to patch.
- * @param {string} filename Source name, for error messages.
- * @return {string} The patched source code.
+ * @param {Object} node AST node.
+ * @return {boolean} True when the node matches.
  */
-export function patchReactLegacyElement( code, filename = 'input.js' ) {
-	const ast = parse( code, { sourceType: 'unambiguous' } );
-	const ms = new MagicString( code );
+function isPropsRefMember( node ) {
+	return (
+		!! node &&
+		node.type === 'MemberExpression' &&
+		! node.computed &&
+		node.property.type === 'Identifier' &&
+		node.property.name === 'ref' &&
+		node.object.type === 'MemberExpression' &&
+		! node.object.computed &&
+		node.object.property.type === 'Identifier' &&
+		node.object.property.name === 'props'
+	);
+}
+
+/**
+ * Returns the `<object>.props.ref` member expression when the statement is an
+ * assignment of the form `x = y.props.ref` (or `var x = y.props.ref`).
+ *
+ * @param {Object} statement AST statement node.
+ * @return {Object|undefined} The `props.ref` member expression, if matched.
+ */
+function getPropsRefAssignment( statement ) {
+	if ( ! statement ) {
+		return undefined;
+	}
+	if ( statement.type === 'ExpressionStatement' ) {
+		let expression = statement.expression;
+		// Minifiers join consecutive statements into a single comma sequence;
+		// the "first statement" is then the first expression of the sequence.
+		if ( expression.type === 'SequenceExpression' ) {
+			expression = expression.expressions[ 0 ];
+		}
+		if (
+			expression.type === 'AssignmentExpression' &&
+			expression.operator === '=' &&
+			isPropsRefMember( expression.right )
+		) {
+			return expression.right;
+		}
+		return undefined;
+	}
+	if (
+		statement.type === 'VariableDeclaration' &&
+		statement.declarations.length === 1 &&
+		isPropsRefMember( statement.declarations[ 0 ].init )
+	) {
+		return statement.declarations[ 0 ].init;
+	}
+	return undefined;
+}
+
+/**
+ * Patches React DOM's `coerceRef` so it falls back to the legacy
+ * `element.ref` when `element.props.ref` is absent. Elements created by older
+ * React runtimes keep the ref on the element itself instead of in props.
+ *
+ * `coerceRef` is identified structurally: its first body statement assigns
+ * `<id>.props.ref` (e.g. `t = t.props.ref`). Bundles without such a function
+ * (e.g. the `react` bundle) are left untouched, and more than one match is
+ * treated as an error since the assumption no longer holds.
+ *
+ * @param {Object}      ast      Parsed AST (File node).
+ * @param {MagicString} ms       Magic string for the source being edited.
+ * @param {string}      filename Source name, for error messages.
+ */
+function patchCoerceRef( ast, ms, filename ) {
+	const matches = [];
+
+	traverse( ast, {
+		Function( path ) {
+			const body = path.node.body;
+			if (
+				! body ||
+				body.type !== 'BlockStatement' ||
+				body.body.length === 0
+			) {
+				return;
+			}
+			const propsRef = getPropsRefAssignment( body.body[ 0 ] );
+			if ( propsRef ) {
+				matches.push( propsRef );
+			}
+		},
+	} );
+
+	if ( matches.length === 0 ) {
+		return;
+	}
+	if ( matches.length > 1 ) {
+		throw new Error(
+			`[react-legacy-element] Expected exactly one \`coerceRef\`-like function (first statement \`x = y.props.ref\`) in ${ filename }, found ${ matches.length }.`
+		);
+	}
+
+	const propsRef = matches[ 0 ];
+	const object = propsRef.object.object;
+	const propsRefText = ms.original.slice( propsRef.start, propsRef.end );
+	const objectText = ms.original.slice( object.start, object.end );
+	ms.overwrite(
+		propsRef.start,
+		propsRef.end,
+		`(${ propsRefText } ?? ${ objectText }.ref)`
+	);
+}
 
+/**
+ * Patches every `Symbol.for('react.transitional.element')` variable so that
+ * checks against it also accept the legacy `Symbol.for('react.element')`.
+ *
+ * @param {Object}      ast      Parsed AST (File node).
+ * @param {MagicString} ms       Magic string for the source being edited.
+ * @param {string}      filename Source name, for error messages.
+ */
+function patchTransitionalSymbols( ast, ms, filename ) {
 	const declarators = [];
 	traverse( ast, {
 		VariableDeclarator( path ) {
@@ -202,6 +311,21 @@ export function patchReactLegacyElement( code, filename = 'input.js' ) {
 	for ( const declPath of declarators ) {
 		patchTransitionalDeclarator( declPath, ms, filename );
 	}
+}
+
+/**
+ * Applies the legacy-element codemod to a bundled React script.
+ *
+ * @param {string} code     Source code to patch.
+ * @param {string} filename Source name, for error messages.
+ * @return {string} The patched source code.
+ */
+export function patchReactLegacyElement( code, filename = 'input.js' ) {
+	const ast = parse( code, { sourceType: 'unambiguous' } );
+	const ms = new MagicString( code );
+
+	patchTransitionalSymbols( ast, ms, filename );
+	patchCoerceRef( ast, ms, filename );
 
 	return ms.toString();
 }
diff --git a/tools/react-19/react-dom.js b/tools/react-19/react-dom.js
index 30bc3d0ce81cb0..f8aee02880823b 100644
--- a/tools/react-19/react-dom.js
+++ b/tools/react-19/react-dom.js
@@ -2,4 +2,5 @@ module.exports = {
 	...require( 'react-dom' ),
 	...require( 'react-dom/client' ),
 	...require( './react-polyfill' ),
+	__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: {},
 };
diff --git a/tools/react-19/react-polyfill.ts b/tools/react-19/react-polyfill.js
similarity index 79%
rename from tools/react-19/react-polyfill.ts
rename to tools/react-19/react-polyfill.js
index 0ff6b154dacf99..e4d185a5b19091 100644
--- a/tools/react-19/react-polyfill.ts
+++ b/tools/react-19/react-polyfill.js
@@ -1,6 +1,5 @@
 import { flushSync } from 'react-dom';
 import { createRoot, hydrateRoot } from 'react-dom/client';
-import type { Root } from 'react-dom/client';
 
 const internalsKey = '_reactInternals';
 
@@ -8,7 +7,7 @@ const internalsKey = '_reactInternals';
 const HostComponent = 5;
 const HostText = 6;
 
-function findCurrentFiber( fiber: any ): any {
+function findCurrentFiber( fiber ) {
 	if ( ! fiber.alternate ) {
 		// First mount — only one version exists, and it's current.
 		return fiber;
@@ -30,7 +29,7 @@ function findCurrentFiber( fiber: any ): any {
 	return fiber.alternate;
 }
 
-function findHostFiber( fiber: any ): any {
+function findHostFiber( fiber ) {
 	const current = findCurrentFiber( fiber );
 	if ( ! current ) {
 		return null;
@@ -38,7 +37,7 @@ function findHostFiber( fiber: any ): any {
 	return findHostFiberImpl( current );
 }
 
-function findHostFiberImpl( fiber: any ): any {
+function findHostFiberImpl( fiber ) {
 	if ( fiber.tag === HostComponent || fiber.tag === HostText ) {
 		return fiber;
 	}
@@ -55,13 +54,13 @@ function findHostFiberImpl( fiber: any ): any {
 	return null;
 }
 
-export function findDOMNode( instance: any ): Element | Text | null {
+export function findDOMNode( instance ) {
 	if ( instance === null || instance === undefined ) {
 		return null;
 	}
 
 	if ( instance.nodeType !== undefined ) {
-		return instance as Element | Text;
+		return instance;
 	}
 
 	const fiber = instance[ internalsKey ];
@@ -79,13 +78,9 @@ export function findDOMNode( instance: any ): Element | Text | null {
 	return hostFiber?.stateNode ?? null;
 }
 
-const roots = new WeakMap< Element, Root >();
+const roots = new WeakMap();
 
-export function render(
-	element: React.ReactNode,
-	container: Element,
-	callback?: () => void
-): void {
+export function render( element, container, callback ) {
 	let root = roots.get( container );
 	if ( ! root ) {
 		root = createRoot( container );
@@ -101,11 +96,7 @@ export function render(
 	}
 }
 
-export function hydrate(
-	element: React.ReactNode,
-	container: Element,
-	callback?: () => void
-): void {
+export function hydrate( element, container, callback ) {
 	let root = roots.get( container );
 	if ( ! root ) {
 		root = hydrateRoot( container, element );
@@ -119,7 +110,7 @@ export function hydrate(
 	}
 }
 
-export function unmountComponentAtNode( container: Element ): boolean {
+export function unmountComponentAtNode( container ) {
 	const root = roots.get( container );
 	if ( ! root ) {
 		return false;
diff --git a/tools/react-19/react.js b/tools/react-19/react.js
index 4fb3922241a923..6f463bc6af3d97 100644
--- a/tools/react-19/react.js
+++ b/tools/react-19/react.js
@@ -4,5 +4,9 @@ module.exports = {
 	__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: {
 		ReactCurrentOwner: { current: null },
 		ReactCurrentDispatcher: { current: null },
+		ReactDebugCurrentFrame: {
+			setExtraStackFrame: () => {},
+			getStackAddendum: () => '',
+		},
 	},
 };