[rfc] Insert empty text node during hydration

packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js

@@ -2958,4 +2958,44 @@ describe('ReactDOMServerPartialHydration', () => {
     expect(ref.current).toBe(span);
     expect(ref.current.innerHTML).toBe('Hidden child');
   });
+
+  function itHydratesWithoutMismatch(msg, App) {
+    it(msg + ' without mismatch', () => {
+      const container = document.createElement('div');
+      document.body.appendChild(container);
+      const finalHTML = ReactDOMServer.renderToString(<App />);
+      container.innerHTML = finalHTML;
+
+      ReactDOM.hydrateRoot(container, <App />);
+      Scheduler.unstable_flushAll();
+    });
+  }
+
+  itHydratesWithoutMismatch('can hydrate empty string ', function App() {
+    return (
+      <div>
+        <div id="test">Test</div>
+        {'' && <div>Test</div>}
+        <div>Test</div>
+      </div>
+    );
+  });
+
+  itHydratesWithoutMismatch('can hydrate empty string simple', function App() {
+    return '';
+  });
+  itHydratesWithoutMismatch('can hydrate empty string simple', function App() {
+    return (
+      <>
+        {''}
+        {'sup'}
+      </>
+    );
+  });
+  itHydratesWithoutMismatch(
+    'can hydrate empty string without mismatch simple 2',
+    function App() {
+      return <Suspense>{'' && false}</Suspense>;
+    },
+  );
 });

packages/react-dom/src/client/ReactDOMHostConfig.js

@@ -692,14 +692,33 @@ export function canHydrateTextInstance(
   instance: HydratableInstance,
   text: string,
 ): null | TextInstance {
-  if (text === '' || instance.nodeType !== TEXT_NODE) {
-    // Empty strings are not parsed by HTML so there won't be a correct match here.
+  if (
+    (instance.textContent !== '' && text === '') ||
+    instance.nodeType !== TEXT_NODE
+  ) {
     return null;
   }
   // This has now been refined to a text node.
   return ((instance: any): TextInstance);
 }
 
+export function insertMissingEmptyTextNode(
+  instance: null | HydratableInstance,
+  parent: null | HydratableInstance,
+): null | HydratableInstance {
+  const parentNode = instance ? instance.parentNode : parent;
+  if (parentNode) {
+    const textNode = document.createTextNode('');
+    if (instance) {
+      parentNode.insertBefore(textNode, instance);
+    } else {
+      parentNode.appendChild(textNode);
+    }
+    return (textNode: TextInstance);
+  }
+  return null;
+}
+
 export function canHydrateSuspenseInstance(
   instance: HydratableInstance,
 ): null | SuspenseInstance {

packages/react-reconciler/src/ReactFiberHostConfigWithNoHydration.js

@@ -23,6 +23,7 @@ export type SuspenseInstance = mixed;
 export const supportsHydration = false;
 export const canHydrateInstance = shim;
 export const canHydrateTextInstance = shim;
+export const insertMissingEmptyTextNode = shim;
 export const canHydrateSuspenseInstance = shim;
 export const isSuspenseInstancePending = shim;
 export const isSuspenseInstanceFallback = shim;

packages/react-reconciler/src/ReactFiberHydrationContext.new.js

@@ -24,6 +24,7 @@ import {
   HostComponent,
   HostText,
   HostRoot,
+  HostPortal,
   SuspenseComponent,
 } from './ReactWorkTags';
 import {ChildDeletion, Placement, Hydrating} from './ReactFiberFlags';
@@ -61,6 +62,7 @@ import {
   didNotFindHydratableInstance,
   didNotFindHydratableTextInstance,
   didNotFindHydratableSuspenseInstance,
+  insertMissingEmptyTextNode,
 } from './ReactFiberHostConfig';
 import {
   enableClientRenderFallbackOnHydrationMismatch,
@@ -327,6 +329,36 @@ function tryHydrate(fiber, nextInstance) {
   }
 }
 
+function tryHydrateEmptyTextNode(
+  fiber: Fiber,
+  nextInstance: null | HydratableInstance,
+  parentFiber: null | Fiber,
+) {
+  if (
+    nextInstance &&
+    canHydrateTextInstance(nextInstance, fiber.pendingProps)
+  ) {
+    return nextInstance;
+  } else {
+    if (!parentFiber) {
+      return null;
+    }
+    switch (parentFiber.tag) {
+      case HostRoot:
+      case HostPortal:
+        return insertMissingEmptyTextNode(
+          nextInstance,
+          parentFiber.stateNode.containerInfo,
+        );
+      case HostComponent:
+        return insertMissingEmptyTextNode(nextInstance, parentFiber.stateNode);
+      default:
+        // Recurse upwards to find parent host node for text node
+        return tryHydrateEmptyTextNode(fiber, nextInstance, parentFiber.return);
+    }
+  }
+}
+
 function throwOnHydrationMismatchIfConcurrentMode(fiber: Fiber) {
   if (
     enableClientRenderFallbackOnHydrationMismatch &&
@@ -342,6 +374,13 @@ function tryToClaimNextHydratableInstance(fiber: Fiber): void {
   if (!isHydrating) {
     return;
   }
+  if (fiber.tag === HostText && fiber.pendingProps === '') {
+    nextHydratableInstance = tryHydrateEmptyTextNode(
+      fiber,
+      nextHydratableInstance,
+      hydrationParentFiber,
+    );
+  }
   let nextInstance = nextHydratableInstance;
   if (!nextInstance) {
     throwOnHydrationMismatchIfConcurrentMode(fiber);

packages/react-reconciler/src/ReactFiberHydrationContext.old.js

@@ -24,6 +24,7 @@ import {
   HostComponent,
   HostText,
   HostRoot,
+  HostPortal,
   SuspenseComponent,
 } from './ReactWorkTags';
 import {ChildDeletion, Placement, Hydrating} from './ReactFiberFlags';
@@ -61,6 +62,7 @@ import {
   didNotFindHydratableInstance,
   didNotFindHydratableTextInstance,
   didNotFindHydratableSuspenseInstance,
+  insertMissingEmptyTextNode,
 } from './ReactFiberHostConfig';
 import {
   enableClientRenderFallbackOnHydrationMismatch,
@@ -327,6 +329,36 @@ function tryHydrate(fiber, nextInstance) {
   }
 }
 
+function tryHydrateEmptyTextNode(
+  fiber: Fiber,
+  nextInstance: null | HydratableInstance,
+  parentFiber: null | Fiber,
+) {
+  if (
+    nextInstance &&
+    canHydrateTextInstance(nextInstance, fiber.pendingProps)
+  ) {
+    return nextInstance;
+  } else {
+    if (!parentFiber) {
+      return null;
+    }
+    switch (parentFiber.tag) {
+      case HostRoot:
+      case HostPortal:
+        return insertMissingEmptyTextNode(
+          nextInstance,
+          parentFiber.stateNode.containerInfo,
+        );
+      case HostComponent:
+        return insertMissingEmptyTextNode(nextInstance, parentFiber.stateNode);
+      default:
+        // Recurse upwards to find parent host node for text node
+        return tryHydrateEmptyTextNode(fiber, nextInstance, parentFiber.return);
+    }
+  }
+}
+
 function throwOnHydrationMismatchIfConcurrentMode(fiber: Fiber) {
   if (
     enableClientRenderFallbackOnHydrationMismatch &&
@@ -342,6 +374,13 @@ function tryToClaimNextHydratableInstance(fiber: Fiber): void {
   if (!isHydrating) {
     return;
   }
+  if (fiber.tag === HostText && fiber.pendingProps === '') {
+    nextHydratableInstance = tryHydrateEmptyTextNode(
+      fiber,
+      nextHydratableInstance,
+      hydrationParentFiber,
+    );
+  }
   let nextInstance = nextHydratableInstance;
   if (!nextInstance) {
     throwOnHydrationMismatchIfConcurrentMode(fiber);

packages/react-reconciler/src/forks/ReactFiberHostConfig.custom.js

@@ -134,6 +134,8 @@ export const cloneHiddenTextInstance = $$$hostConfig.cloneHiddenTextInstance;
 // -------------------
 export const canHydrateInstance = $$$hostConfig.canHydrateInstance;
 export const canHydrateTextInstance = $$$hostConfig.canHydrateTextInstance;
+export const insertMissingEmptyTextNode =
+  $$$hostConfig.insertMissingEmptyTextNode;
 export const canHydrateSuspenseInstance =
   $$$hostConfig.canHydrateSuspenseInstance;
 export const isSuspenseInstancePending =