Problem/Motivation
LanguageNegotiationUrl::processOutbound() includes the manipulation of the 'absolute' option:
<?php
// Ask for an absolute URL with our modified base URL.
$options['absolute'] = TRUE;
$options['base_url'] = $url_scheme . '://' . $config['domains'][$options['language']->getId()];
// ...
?>
Leading to the situation that if you want to generate a Url string, e.g. from an entity like this:
<?php
$entity->toUrl()->setAbsolute(FALSE)->toString();
?>
... you'll end up having an absolute Url string regardless your previously set option.
This can be a huge problem, as it leads to unexpected (and most probably untested) behavior where errors might not be noticed immediately, but after some period of time when things might be just too late to fix them.
Steps to reproduce
I'll try to find time to be more concise about how to reproduce this problem. Basically you need a multilingual site with domains configured accordingly, then having some part in your code that wants to have a relative Url.
Proposed resolution
Maybe it would help to fail hard by throwing an exception, when the 'absolute' option was explicitly set to FALSE before.
Remaining tasks
User interface changes
API changes
Data model changes
Release notes snippet