Drupal\Core\Asset\CssCollectionOptimizer is deprecated!

Let's try this, to get started...

Let's fix some tests...

Fix coding standard violation.

Setting to NR; also requires manual testing against real sites.

Did not work. Tested and found that works sometimes due to how HttpKernel works. The first request file is created and returns an instance of Response, afterward it returns an instance of BinaryFileResponse because the file already exists on file system.

// Since Drupal 10, assets are lazily constructed.
$sub_request = Request::create($asset['data']);

$raw_request = $this->httpKernel->handle($sub_request, HttpKernelInterface::SUB_REQUEST);

if ($raw_request instanceof \Symfony\Component\HttpFoundation\BinaryFileResponse) {
    $css_request = $raw_request->getFile()->getContent();
}
else {
    $css_request = $raw_request->getContent();
}

    $css .= $css_request;

Thanks for the manual test!

It sounds like it's going to be hard to kernel test this...almost sounds race condition-ny...maybe we'll have to go heavier into functional test territory to land this.

I think it would be more maintainable long term to copy some of the logic in AssetControllerBase::deliver() (can be massively slimmed down because it doesn't need all the request validation) - build what would have been the filename contents from the contents of the $css_assets array, and store it in the Drupal object cache rather than as a file.