file_url_transform_relative() only works with current request's host

file_url_transform_relative() was deprecated in 9.3.0 and was converted to \Drupal\Core\File\FileUrlGeneratorInterface::transformRelative(). At the very least this needs an IS update. Can we check if this is still an issue in the new code?

I was following the issue. The code has changed a little bit, but not substantially that would solve the root cause in the public method.

I went through some of the use cases and issues.

- #2257291: Handle alternate domains in filter_html_image_secure → - still applies
- #2825593: Allow relative urls for favicons in config → - as an example of a contrib theme/module storing a URL into config, but I don't think this applies as I don't think that theme should have been storing a module/theme provided file in config.
- ImageUrlFormatter / ImageStyleInterface::buildUrl - does not apply as that uses stream wrappers.
- ::transformRelative is a public method so it would be possible for custom and contrib code to pass in an absolute URL similar to what is in core filter module.

So if we added a mismatch of URL and host to UrlTransformRelativeTest::providerFileUrlTransformRelative, then we would
probably get failures.

For a functional test, I'm not sure if we could modify FilterHtmlImageSecureTest::testImageSource since we probably would want to mock the the request host so maybe a separate functional test in that file?

There is now a workaround however - override the file_url_generator service, but I don't think we want to recommend that.

I'm leaving the issue tags and status as-is.