TYPO3 Core

I would suggest that the fix is, that it searches for all local storages and takes their prefixes and searches through them in the same place - what do you think?

The method TypoScriptFrontendController::setAbsRefPrefix() already loops over the storages and sets the absRefPrefix for all folders:
https://github.com/TYPO3/TYPO3.CMS/blob/2ba08444c9cd7613c97e10b65fd53867565f205c/typo3/sysext/frontend/Classes/Controller/TypoScriptFrontendController.php#L3884

There it performs a simple str_replace, where it takes the prefix of each storage and prepends a quote. So it searches for "fileadmin instead of only fileadmin. This works in most cases, when the path directly appears in an attribute, but not when it appears at other places.

It's not possible to perform a simple str_replace, when you leave the quote, because you would also replace fileadmin where it already is prepended by a prefix or a domain. I think this would quickly introduce new bugs. That's why I would only change the getPublicUrl()-method of ResourceStorage. If the introduces coupling between ResourceStorage and TSFE is undesired, there could be a hook like emitPostGeneratePublicUrlSignal and TSFE fixes the paths within the hook.

I am using Smarty as template engine, and work directly with FileReference-Objects within the template and call their getPublicUrl-method ;)

But either way, it's not restricted to srcset. Anywhere, where an integrator uses the publicUrl, and it is not at the start of an attribute, absRefPrefix will not work.

Another example is a background image:

lib.backgroundImage = IMG_RESOURCE
lib.backgroundImage.file.import.data = levelmedia:-1, slide
lib.backgroundImage.file.import.listNum = 0
lib.backgroundImage.file.treatIdAsReference = 1
lib.backgroundImage.stdWrap.wrap (
  <div style="background-image:url('|');" class="page-background"></div>
)

page.20 < lib.backgroundImage

Here the publicUrl is not prefixed with the absRefPrefix.

Other examples might be, if you want to output a url within JavaScript, which gets generated by TypoScript.

I also discovered the problem in TYPO3 8.4.0-dev (master) within the `f:uri.resource` view helper. Is this intentionally, or even a relevant error?

I also encountered this bug setting in a Fluid partial:

style="background-image: url('{f:cObject(typoscriptObjectPath: 'lib.imagePlaceholderFluid', data: {image: item.backgroundImage.0.uid})}');" 

url(

Depending on need the usage of absRefPrefix might be wrong here.
IMO there should be either a parameter if absRefPrefix shall be prepended or a second variable including the merged path.
As the usage is in different places perhaps the parameter solution is easier in realization and usage.
So a general approach to change the paths without options is wrong in my opinion.

I tried to repeat the test reported on #82486 on a 9.5.10 TYPO3 installation, i.e.

1) TS Setup

config {
  absRefPrefix = /foo/
  compressJs = 1
}

page = PAGE
page {
  10 = TEXT
  10.value = HELLO WORLD!
  # setting a link to test if absRefPrefix works
  10.typolink.parameter = 1

  includeJSLibs.js1 = fileadmin/js1.js
  includeJSLibs.js2 = fileadmin/js2.js

  includeCSS.css1 = fileadmin/css1.css
  includeCSSLibs.css1 = fileadmin/css1.css
}

Results:¶

The files are prepended with the /foo prefix:

<link rel="stylesheet" type="text/css" href="/foo/fileadmin/css1.css?1571951025" media="all">
<link rel="stylesheet" type="text/css" href="/foo/fileadmin/css1.css?1571951025" media="all">

<script src="/foo/typo3temp/assets/compressed/js1-073ad306084932e4f6f8cc94ecc220df.js?1571951083" type="text/javascript"></script>
<script src="/foo/typo3temp/assets/compressed/js2-5902d8a0726c46b4d9e425058757d08c.js?1571951083" type="text/javascript"></script>

while the HELLO WORLD! link is not (ID=1 is my homepage, but the result does not change with choosing another page):

<a href="/">HELLO WORLD!</a>

Any workaround? I really need this. A customer imports the HTML and needs absolute paths.