Internal links are missing their base path

> In tests, though, the base path is being added correctly.

Is there any way we could come up with a failing test st least. Based on the description that sounds possible?

Could you write up steps to reproduce on a clean install?

I don't know if a failing test would be possible. What the MR does, is to set the base URL always as this:

$baseUrl = $this->request->getSchemeAndHttpHost() . $this->request->getBasePath();

The trailing $this->request->getBasePath() is missing so far. But the 2 test classes LinkcheckerLinkExtractorServiceTest and LinkcheckerRepair301Test have already defined the base path in the correct format including the trailing $this->request->getBasePath().

In "normal" installations, that doesn't make any difference, as $this->request->getBasePath() is empty anyways. But if Drupal is installed in a sub-directory, so that you have to call the front-page as https://www.example.com/subpath, i.e. the index.php is in that subdirectory and not in the web root, then /subpath is what $this->request->getBasePath() return. To make it even more clear, the URL for node 1 would then be https://www.example.com/subpath/node/1.

As this scenario requires a webserver setup accordingly, I don't know if something like that could be covered by our test environment.