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:

// CORRECT React 19 root (same as React 18):
import { createRoot } from 'react-dom/client';
const root = createRoot(document.getElementById('root'));