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plugins/react19-upgrade/skills/react19-concurrent-patterns/SKILL.md

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+---
+name: react19-concurrent-patterns
+description: 'Preserve React 18 concurrent patterns and adopt React 19 APIs (useTransition, useDeferredValue, Suspense, use(), useOptimistic, Actions) during migration.'
+---
+
+# React 19 Concurrent Patterns
+
+React 19 introduced new APIs that complement the migration work. This skill covers two concerns:
+
+1. **Preserve**  existing React 18 concurrent patterns that must not be broken during migration
+2. **Adopt**  new React 19 APIs worth introducing after migration stabilizes
+
+## Part 1  Preserve: React 18 Concurrent Patterns That Must Survive the Migration
+
+These patterns exist in React 18 codebases and must not be accidentally removed or broken:
+
+### createRoot  Already Migrated by the R18 Orchestra
+
+If the R18 orchestra already ran, `ReactDOM.render` → `createRoot` is done. Verify it's correct:
+
+```jsx
+// CORRECT React 19 root (same as React 18):
+import { createRoot } from 'react-dom/client';
+const root = createRoot(document.getElementById('root'));
+root.render(
+  <React.StrictMode>
+    <App />
+  </React.StrictMode>
+);
+```
+
+### useTransition  No Migration Needed
+
+`useTransition` from React 18 works identically in React 19. Do not touch these patterns during migration:
+
+```jsx
+// React 18 useTransition  unchanged in React 19:
+const [isPending, startTransition] = useTransition();
+
+function handleClick() {
+  startTransition(() => {
+    setFilteredResults(computeExpensiveFilter(input));
+  });
+}
+```
+
+### useDeferredValue  No Migration Needed
+
+```jsx
+// React 18 useDeferredValue  unchanged in React 19:
+const deferredQuery = useDeferredValue(query);
+```
+
+### Suspense for Code Splitting  No Migration Needed
+
+```jsx
+// React 18 Suspense with lazy  unchanged in React 19:
+const LazyComponent = React.lazy(() => import('./LazyComponent'));
+
+function App() {
+  return (
+    <Suspense fallback={<Spinner />}>
+      <LazyComponent />
+    </Suspense>
+  );
+}
+```
+
+---
+
+## Part 2  React 19 New APIs
+
+These are worth adopting in a post-migration cleanup sprint. Do not introduce these DURING the migration  stabilize first.
+
+For full patterns on each new API, read:
+- **`references/react19-use.md`**  the `use()` hook for promises and context
+- **`references/react19-actions.md`**  Actions, useActionState, useFormStatus, useOptimistic
+- **`references/react19-suspense.md`**  Suspense for data fetching (the new pattern)
+
+## Migration Safety Rules
+
+During the React 19 migration itself, these concurrent-mode patterns must be **left completely untouched**:
+
+```bash
+# Verify nothing touched these during migration:
+grep -rn "useTransition\|useDeferredValue\|Suspense\|startTransition" \
+  src/ --include="*.js" --include="*.jsx" | grep -v "\.test\."
+```
+
+If the migrator touched any of these files, review the changes  the migration should only have modified React API surface (forwardRef, defaultProps, etc.), never concurrent mode logic.