Server islands render empty in dev when using a custom src/fetch.ts (renderers not loaded for built-in routes)

Astro Info

Astro                    v7.3.3
Vite                     v8.3.0
Node                     v25.0.0
System                   macOS (arm64)
Package Manager          npm
Output                   server
Adapter                  @astrojs/node (v11.1.6)
Integrations             @astrojs/svelte (v9.0.1)

If this issue only occurs in one browser, which browser is a problem?

Safari (Chromium usually hides it, see below). Reproducible in any browser via the script in the repro.

Describe the Bug

In astro dev, a server island containing a framework component (React, Svelte, Vue, …) renders empty when the project has a custom src/fetch.ts built on astro/fetch.

• The /_server-islands/<Name> response contains no component HTML, so the client replaces the fallback with nothing and leaves a blank hole.
• The dev server logs [NoMatchingRenderer] Unable to render 'Message' even though the integration is configured.
• Delete src/fetch.ts and the same island renders correctly.
• Production builds are unaffected.

Why it happens:

• FetchState (astro/fetch) reads the manifest via getAmbientManifest(), i.e. the virtual:astro:manifest module export, rather than the stable app.manifest object.
• virtual:astro:manifest depends on virtual:astro:server-island-manifest, which the server islands plugin invalidates on every .astro/.mdx transform. Rendering the page invalidates it, so the next request gets a fresh manifest object with no renderers.
• In createRunnableEnvironment (vite-plugin-app/environment.ts), getComponentByRoute returns early for built-in routes like /_server-islands/[name] before loading renderers into devRenderers. Renderers are only loaded when a user route renders, and only onto whichever manifest object that request used.
• The island request is a built-in route on a fresh manifest → zero renderers → NoMatchingRenderer.

Why it looks browser-specific:

• Chromium honours <link rel="preload" as="fetch"> for the island URL, so the island request starts while the page render is still in flight and shares its manifest. Works.
• Safari ignores that preload and only requests the island from the inline <script> after page load. By then the manifest has been invalidated. Blank.

What's the expected result?

• The island renders the framework component in dev regardless of how the manifest is obtained or when the island request arrives.
• Renderers should be loaded for every request reaching the runnable environment, including built-in routes.

Fix proposed in #18000: move the renderer-loading block ahead of the built-in route check in getComponentByRoute.

Link to Minimal Reproducible Example

https://github.com/barclayd/astro-island-repro

npm install && npx astro dev, then npm run repro in another terminal (fetches the page, then the island, like Safari does). Prints FAIL on 7.3.3; delete src/fetch.ts and it prints PASS.

Participation

• I am willing to submit a pull request for this issue.