Don't cut off effects at end of list if hydrating (#18872)

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

@@ -1699,7 +1699,7 @@ describe('ReactDOMServerPartialHydration', () => {
   });
 
   // @gate experimental
-  it('clears server boundaries when SuspenseList does a second pass', async () => {
+  it('clears server boundaries when SuspenseList runs out of time hydrating', async () => {
     let suspend = false;
     let resolve;
     const promise = new Promise(resolvePromise => (resolve = resolvePromise));
@@ -1791,6 +1791,63 @@ describe('ReactDOMServerPartialHydration', () => {
     expect(ref.current).toBe(b);
   });
 
+  // @gate experimental
+  it('clears server boundaries when SuspenseList suspends last row hydrating', async () => {
+    let suspend = false;
+    let resolve;
+    const promise = new Promise(resolvePromise => (resolve = resolvePromise));
+
+    function Child({children}) {
+      if (suspend) {
+        throw promise;
+      } else {
+        return children;
+      }
+    }
+
+    function App() {
+      return (
+        <Suspense fallback={null}>
+          <SuspenseList revealOrder="forwards" tail="hidden">
+            <Suspense fallback="Loading A">
+              <span>A</span>
+            </Suspense>
+            <Suspense fallback="Loading B">
+              <Child>
+                <span>B</span>
+              </Child>
+            </Suspense>
+          </SuspenseList>
+        </Suspense>
+      );
+    }
+
+    suspend = true;
+    const html = ReactDOMServer.renderToString(<App />);
+
+    const container = document.createElement('div');
+    container.innerHTML = html;
+
+    const root = ReactDOM.createRoot(container, {hydrate: true});
+
+    suspend = true;
+
+    await act(async () => {
+      root.render(<App />);
+    });
+
+    // We haven't hydrated the second child but the placeholder is still in the list.
+    expect(container.textContent).toBe('ALoading B');
+
+    suspend = false;
+    await act(async () => {
+      // Resolve the boundary to be in its resolved final state.
+      await resolve();
+    });
+
+    expect(container.textContent).toBe('AB');
+  });
+
   // @gate experimental
   it('can client render nested boundaries', async () => {
     let suspend = false;

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

@@ -1098,7 +1098,8 @@ function completeWork(
             if (
               renderState.tail === null &&
               renderState.tailMode === 'hidden' &&
-              !renderedTail.alternate
+              !renderedTail.alternate &&
+              !getIsHydrating() // We don't cut it if we're hydrating.
             ) {
               // We need to delete the row we just rendered.
               // Reset the effect list to what it was before we rendered this

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

@@ -1114,7 +1114,8 @@ function completeWork(
             if (
               renderState.tail === null &&
               renderState.tailMode === 'hidden' &&
-              !renderedTail.alternate
+              !renderedTail.alternate &&
+              !getIsHydrating() // We don't cut it if we're hydrating.
             ) {
               // We need to delete the row we just rendered.
               // Reset the effect list to what it was before we rendered this