invalid static requests return index.html instead of 404

[eltigerchino]

Describe the bug

If the home page / is prerendered, invalid static requests (those matching the paths in _routes.json excluded but are not found) such as /_app/abcd, incorrectly return a 200 status code and the content of the prerendered /index.html file if any.

However, if a 404.html file is found, it is returned alongside the expected 404 status code.

Reproduction

1. Find a Cloudflare Pages website deployed with @sveltejs/adapter-cloudflare.
2. Navigate to a non-existent /_app/* URL.

For example, the URL below should return a 404 but returns the prerendered home page:

https://brgh.church/_app/doesntexist

Logs

No response

System Info

System:
    OS: macOS 13.2
    CPU: (8) arm64 Apple M1 Pro
    Memory: 142.81 MB / 32.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 16.18.0 - ~/.nvm/versions/node/v16.18.0/bin/node
    Yarn: 3.2.3 - ~/.nvm/versions/node/v16.18.0/bin/yarn
    npm: 8.19.2 - ~/.nvm/versions/node/v16.18.0/bin/npm
  Browsers:
    Chrome: 110.0.5481.77
    Chrome Canary: 112.0.5586.0
    Firefox: 109.0.1
    Firefox Developer Edition: 110.0
    Safari: 16.3
    Safari Technology Preview: 16.4
  npmPackages:
    @sveltejs/adapter-cloudflare: ^2.0.1 => 2.0.1 
    @sveltejs/kit: ^1.3.10 => 1.3.10 
    svelte: ^3.55.1 => 3.55.1 
    vite: ^4.1.1 => 4.1.1

Severity

annoyance

Additional Information

When the root page is not prerendered, invalid requests to /_app/* return a blank page and a 404.
So, should the Cloudflare adapter include a /404.html by default to ensure this occurs consistently?
Or is it enough to document the importance of including a 404.html page?