fix(metadata): prevent layout title template from applying to the same route-layer page

[NriotHrreion]

Closes #3065

Overview

This PR fixes an App Router metadata compatibility issue where vinext applied a leaf layout's title.template to a page on the same route-tree layer. Next.js only propagates that template to deeper child routes.

This restores SSR output parity with Next.js, avoiding related SEO and accessibility regressions.

What changed

Added a field stashesTitleTemplate in MetadataMergeEntry to mark the entries on the same route-tree layer, preventing them from applying the layout's title template.

In packages/vinext/src/server/app-page-head.ts, filter the same route-layer pages.

const metadataEntries: MetadataMergeEntry[] = [
  ...layoutMetadataResults.flatMap((metadata, index) => {
    if (!metadata) return [];

    const isSameLayerAsPage =
      hasPrimaryPageLayer && layoutInputs[index]?.treePosition === routeSegments.length;
    return [
      {
        metadata,
        ...(isSameLayerAsPage ? { stashesTitleTemplate: false } : {}),
      },
    ];
  }),
  // ...other entries
];

Testing

• pnpm test tests/app-page-head.test.ts tests/nextjs-compat/metadata.test.ts
• playwright test tests/e2e/app-router/nextjs-compat/metadata.spec.ts --project=app-router

[pkg-pr-new]

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commit: b825d90

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Suppress same-layer templates in parallel route branches

When the primary page has no title and an active parallel slot supplies the title, this check only marks layouts from the primary layoutInputs. A slot whose leaf layout and page share a tree position is flattened from resolveParallelRouteMetadata without stashesTitleTemplate: false, so the slot layout still wraps its same-layer page title—for example, producing Slot page | Slot instead of Slot page. Apply the same layer-aware suppression while constructing parallel-route merge entries as well.

AGENTS.md reference: AGENTS.md:L84-L86

Useful? React with 👍 / 👎.

[github-actions]

Performance benchmarks

Compared b825d90 against base 20fdac4 using alternating same-runner rounds. Next.js was unchanged and skipped.

0 improved · 0 regressed · 6 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	141.4 KB	141.4 KB	⚫ +0.0%
Client entry size (gzip)	vinext	128.8 KB	128.8 KB	⚫ +0.0%
Dev server cold start	vinext	3.19 s	3.15 s	⚫ -1.1%
Production build time	vinext	3.66 s	3.64 s	⚫ -0.3%
RSC entry closure size (gzip)	vinext	115.6 KB	115.7 KB	⚫ +0.1%
Server bundle size (gzip)	vinext	196.8 KB	196.9 KB	⚫ +0.0%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5% · paired base/head}