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App Router Repeatedly Retries Hydration While A Client Component Chunk Is Loading

On cold load with a delayed client component chunk, Next.js App Router repeatedly retries root hydration causing thousands of empty commits and significant main thread time. The issue is triggered by `useDeferredValue` calls in `Head` and `InnerLayoutRouter` that pass the current RSC value as the initial value even when no prefetch data exists during initial hydration. This leads to React scheduling deferred work that commits an empty tree, making hydration eligible to retry in a loop.

highConfidence 95%Next.js Affected V16.5.0-Canary.2 Affected V16.3.8

Origin Analysis

`useDeferredValue` in `Head` and `InnerLayoutRouter` is called with a second `initialValue` argument equal to the current value even when `prefetchRsc` is `null` (during initial hydration). React's current implementation treats this as a request to schedule deferred work, which commits an empty tree while the root is suspended waiting for a client component chunk. That empty commit makes hydration eligible to retry, repeating the cycle until the chunk arrives. Underlying React behavior is tracked in react/react#37682.

1. Clone https://github.com/filipzapletalcs/next-head-deferred-hydration-repro 2. Run `npm install && npx playwright install chromium` 3. Build and start production server: `npm run build && npm start` 4. In another terminal run `npm run measure -- --cpu 1 --network none --hold-ms 1500` to artificially delay the `<Heavy />` client component chunk for 1.5 seconds. 5. Observe via React DevTools hook that hydration retries and empty commits occur until the chunk arrives.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This change relies on React 19's `useDeferredValue` semantics where omitting the `initialValue` avoids scheduling a deferred update when no prior value is needed. It is safe for the reported versions but could behave differently in older React versions that do not support the optional second argument or have different deferred scheduling. Rollback is straightforward: revert the two lines to always pass `prefetchRsc` as second argument. No persistence or data loss is involved.