Description

This resumes #31504.
When a pathPrefix is used and the hosting is configured to redirect from the root domain to the pathPrefix (perfectly reasonable behaviour I think), triyng to load a non-existing page breaks React .

Steps to reproduce

Reproduction was provided in said issue and in related discussion #31531.

Expected result

The 404 page should be visible and interactive.

Actual result

The 404 page is loaded but React breaks with "Error: page resources for /xxx not found. Not rendering React.", and the page is not interactive (js is broken).

Investigation

I've debugged the issue carefully, and I've traced down the root cause:
this line:

tries to check a resource existence (comment says "check if html file exist using HEAD request: if it does we should navigate to it instead of showing 404").
The error is that the rawPath it's using, is stripped from the pathPrefix. As I can see, this is used because the internal db of fetched resources is normalized, and anywhere else this function is used: which adds back the pathprefix.
But there the check is done without the pathPrefix, so it calls the incorrect url. This is not usually a problem because it would normally catch another non-existing url, so the result is the same, but with the described hosting behaviour, anything called on the root is redirected on the pathPrefix (with a 302), so this check fails, incorrectly triggering an error.