diff --git a/vite.config.ts b/vite.config.ts
index 15adf71..74467d3 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -99,8 +99,20 @@ export default defineConfig(() => ({
           ) {
             return 'vendor-bitcoin';
           }
-          // React ecosystem — very stable, long cache life
-          if (id.includes('react-dom') || id.includes('react-router')) {
+          // React ecosystem — very stable, long cache life.
+          // IMPORTANT: `react` (core, with useLayoutEffect etc.) MUST live in
+          // the SAME chunk as react-dom / react-router. If react-dom is split
+          // into vendor-react but core `react` lands elsewhere, react-dom runs
+          // before React is defined → "Cannot read properties of undefined
+          // (reading 'useLayoutEffect')". The `/react/` and scheduler matches
+          // below keep the whole React runtime together and loaded first.
+          if (
+            id.includes('node_modules/react/') ||
+            id.includes('node_modules/react-dom/') ||
+            id.includes('node_modules/scheduler/') ||
+            id.includes('react-router') ||
+            id.includes('react/jsx-runtime')
+          ) {
             return 'vendor-react';
           }
           // Radix UI components — large but stable