These make me pretty uneasy. What about an external service dependency that handles Globals, so it can be mocked or replaced? This behavior within a Factory is very counterintuitive.

What do you mean?
Adding another class that handles globals?

    private $superglobals;

    private function updateServer(string $key, string $value): void
    {
        $this->superglobals->setServer($key, $value);
    }

Yes, precisely. It could be adapted to anywhere else in the framework we touch globals.

Added class Superglobals.

isset($this->superglobals->server('SCRIPT_NAME')[0])
What is this condition checked?
$_SERVER['SCRIPT_NAME'] is a string.

The condition comes from:

And it came from:
bcit-ci/CodeIgniter@ead327f7

Does that end up being translated as this, because of string array access?

$_SERVER['SCRIPT_NAME'] !== ''

Yes. I assume this is unintended but that's the equivalence.

https://3v4l.org/XC3NO
https://3v4l.org/hqebU

Refactored. fde3216