Regression of relURL since 0.101.0

[multun]

What version of Hugo are you using (hugo version)?

All versions since 0.101.0 are affected. 0.100.2 is the last known working version.
This breakage was introduced by #10002

Does this issue reproduce with the latest release?

Yes

Description

When a baseURL has a path (like so http://website.com/path/), relURL fails to take the path into account.
This leads to broken links and javascript failing to load with docsy.

Reproducer

# get the site source
git clone --recurse-submodules [email protected]:DGEXSolutions/osrd-website.git
cd osrd-website
# npm install dependencies
docker run --rm -it -v $(pwd):/src -w /src/themes/docsy --entrypoint npm klakegg/hugo:ext-alpine install
# build the website with a working revision
docker run --rm -it -v $(pwd):/src -p 1313:1313 klakegg/hugo:0.100.2-ext-alpine build
grep /js/tabpane-persist.js public/fr/docs/index.html
# build the website with a failing revision
docker run --rm -it -v $(pwd):/src -p 1313:1313 klakegg/hugo:0.101.0-ext-alpine build
grep /js/tabpane-persist.js public/fr/docs/index.html

This reproducers observes the result of <script src='{{ "js/tabpane-persist.js" | relURL }}'></script> with baseURL = "https://dgexsolutions.github.io/osrd-website/"

With the failing revision:

<script src='/js/tabpane-persist.js'></script>

With the working revision:

<script src='/osrd-website/js/tabpane-persist.js'></script>

[jmooring]

See #10049 (comment)

The documentation is correct:
https://gohugo.io/functions/relurl/

[bep]

This was a bug fix, not a regression.

[multun]

@bep The linked issue is indeed a bugfix, but I believe it mistakenly introduced a regression, see my explanation below

@jmooring there must be a misunderstanding, the linked comment and case mentioned applies when an url start with a slash, whereas mine does not: according to documentation, {{ "js/tabpane-persist.js" | relURL }} with baseURL = "https://whatever.com/osrd-website/" should result in /osrd-website/js/tabpane-persist.js

It's also not an issue in my code: it broke good old docsy (at least when some features are enabled, such as the persistent tab pane or swagger UI)

[jmooring]

@multun Let's take further discussion of this to the forum (https://discourse.gohugo.io).