fix: show error.html when root layout load() throws in SPA mode

[Zelys-DFKH]

closes #13721

In SPA mode (ssr: false on the root layout), when load() throws, SvelteKit calls load_root_error_page to show an error page. That function re-runs the same root layout load() to gather data for the error boundary, which throws again. The inner catch had a // TODO: SPA mode? comment and re-threw with no handler. Blank page.

The fix reads error.html at build time and bundles the content into app.js as error_template (via write_client_manifest). When the inner catch fires, it writes the template directly to the document, substituting %sveltekit.status% and %sveltekit.error.message%: the same substitution the server already does. The original throw error is preserved as a fallback if error_template is empty.

The regression test is in the existing no-ssr test app: a /root-layout-error route throws from load(), and Playwright verifies that error.html renders with the right status and message rather than a blank page.

(Thanks to @inq for independently identifying the root cause and the same approach in the issue comments.)

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15798-svelte.vercel.app/

[Zelys-DFKH]

🔔 Gentle ping — this PR has been ready with green CI for several days. Happy to address feedback or answer questions about the approach!

[teemingc]

There are 6 CI checks currently failing

[Zelys-DFKH]

Thanks for the nudge. My fault — the test I added was waiting for body.started, which is the CSS class SvelteKit sets after initialize() finishes. On this code path initialize() never runs (the error gets thrown before it gets there), so the test was always going to time out. Adding { wait_for_started: false } fixes it.

I also swapped out document.open() / document.write() / document.close() for DOMParser + adoptNode. The document.open() call fires executionContextsCleared on Windows, which destroys Playwright's connection to the page — and document.write() is deprecated anyway. The in-place DOM replacement avoids both problems.

The other failures in the current/beta jobs are showing up on main today too, so they're not related to this PR.

[Zelys-DFKH]

Took a shot at this. Since client.js lives in the source tree rather than the generated folder, it can't resolve ./error-template.js directly. Went with a getter in app.js instead: it lives in the generated folder, so the relative path resolves correctly and Vite can follow the static string for chunking.

[Rich-Harris]

thank you!