[MarkusNeusinger]

Summary

• One title and description per page. The SPA shell's default <title>, description, and Open Graph and Twitter tags stayed in <head> next to the ones every page renders since refactor(app): replace react-helmet-async with React 19 native metadata #12424, so each route had two of each and the shell's description came first. The six defaults are now marked data-shell-default, and watchShellDefaults() (a MutationObserver on <head>, started in app/src/main.tsx) detaches a default only while a page renders a tag of the same kind and re-attaches it when none is left. /debug gets its own title.
• How it was built. Best-of-2 run with Google Jules (Gemini Flash) on feature/head-metadata-dedup: variant A deleted the shell defaults (fix(app): drop the shell's per-route head defaults #12439, closed: clients without JavaScript lost title and description, and /debug had none), variant B removed them once at boot (fix(app): strip the shell's head defaults when the app boots #12440, merged into the feature branch). The Gemini Flash pre-PR check then found that B left the tab without a title while a lazy route chunk loads, on the error boundary, and on a loading or failed spec page, and that its tests accepted zero descriptions; Claude replaced the one-time removal with the fallback-keeping sync and tightened the tests.

Plan

Refs #9629 (the head-duplication half of audit Medium#6).

Test plan

• yarn lint, yarn fm:check, yarn type-check, yarn test (110 files, 1192 tests), yarn build
• Built against the production API and loaded in headless Chromium: exactly one <title>, one description and one og:title on /, /plots, /plots?lib=matplotlib, /about, /legal, /debug, /scatter-basic and a 404; /debug and the 404 keep the shell description as their fallback; a client-side navigation from / to /about leaves one title
• Built HTML still carries the shell defaults for clients without JavaScript
• Gemini 3.8 Flash pre-PR check: its title-fallback and test findings fixed; its claim that only SpecPage renders a description was checked and is wrong (eleven pages do)

[Copilot]

🔵 Needs a closer look

Browser-runtime confirmation is still needed because the fix depends on React 19 metadata hoisting and MutationObserver ordering during lazy, error, loading, and client-side route transitions, while the local servers were unavailable.

No open issues 🎉

What changed in this PR

This pull request deduplicates SPA page metadata while preserving shell defaults for non-JavaScript, loading, and error states. It also adds titles for all /debug states and expands metadata synchronization tests.

Changes:

• Marks six shell metadata tags as fallback defaults.
• Adds MutationObserver-based synchronization for route metadata.
• Adds /debug titles across all render states.
• Adds unit, integration, and changelog coverage.

File	Summary
changelog.d/​head-metadata-dedup.md	Documents the metadata deduplication and fallback behavior.
app/​src/​utils/​shellMetadata.ts	Implements shell metadata detection, detachment, restoration, and observation.
app/​src/​utils/​shellMetadata.test.ts	Tests synchronization, restoration, idempotency, and observer behavior.
app/​src/​pages/​headMetadata.test.tsx	Tests page-level metadata deduplication; coverage should also include all supported Open Graph and Twitter tags and shell-only fallback behavior. Nit.
app/​src/​pages/​DebugPage.tsx	Adds titles across authenticated, loading, error, and success states.
app/​src/​main.tsx	Starts shell metadata observation before rendering the React application.
app/​index.html	Marks six shell metadata tags as defaults; og:url and og:image remain unmarked and can duplicate SpecPage metadata. Moderate.

This pull request requires changes.

_{Review effort: Lite}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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