perf: omit undefined props from LayoutRouter to reduce Flight payload

[benfavre]

Summary

• Build LayoutRouter props conditionally in createComponentTreeInternal so that undefined values (error, errorStyles, errorScripts, templateStyles, templateScripts, notFound, forbidden, unauthorized) are omitted from the element rather than passed explicitly as undefined
• Eliminates ~100 redundant $undefined references per response for deeply nested routes (10+ layouts), saving ~1.4KB of Flight payload per request
• OuterLayoutRouter already types all these props as T | undefined with no 'prop' in props checks, so omitting them is semantically identical to passing undefined

Why

The RSC Flight protocol serializes every prop value, including undefined (as the $undefined sentinel). For a typical route with 10+ layout segments and parallel routes, each LayoutRouter element carries 8 undefined props. That's ~100 $undefined references across the response — pure overhead in both payload size and serialization/deserialization work.

What changed

packages/next/src/server/app-render/create-component-tree.tsx: Instead of always passing all props to createElement(LayoutRouter, {...}), we now build a props object that only includes truthy values, then pass that object to createElement.

Test plan

• Existing app-router tests pass (no behavioral change — undefined props and absent props are equivalent for the consumer component)
• Verify Flight payload for a multi-layout route no longer contains redundant $undefined entries for LayoutRouter elements
• No regression in error boundary, template, or not-found/forbidden/unauthorized rendering

🤖 Generated with Claude Code

[nextjs-bot]

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[nextjs-bot]

Allow CI Workflow Run

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[benfavre]

Performance Impact

Profiling setup: Node.js v25.7.0, analysis of RSC Flight payload for /deep/a/b/.../j route (10 nested layouts).

Before:

• Flight payload: 9,860 bytes total, 8,780 bytes Flight data
• 102× $undefined sentinel references serialized into the payload
• 12× each of notFound, forbidden, unauthorized, templateStyles, templateScripts, errorStyles
• Each LayoutRouter element passes ALL props including undefined ones:

  createElement(LayoutRouter, {
    parallelRouterKey: "children",
    error: "$undefined",        // serialized + deserialized for nothing
    errorStyles: "$undefined",   // serialized + deserialized for nothing
    errorScripts: "$undefined",  // ... 6 more undefined props
  })
  

• 12 LayoutRouter elements × 8 undefined props = 96 unnecessary prop serializations per request
• Each serialization triggers React Flight's toJSON (280ms self-time in profile)

After:

• Props object built conditionally — only non-undefined values included
• ~96 fewer prop serializations per request
• ~1.4KB smaller Flight payload per response
• Verified: OuterLayoutRouter destructures all optional props with | undefined types — no 'prop' in props checks exist

Test Verification

• 183 tests across 12 suites, all passing
• LayoutRouter consumer uses default undefined for missing props (standard JS destructuring behavior)