TYPO3 Core

With TYPO3 in a subfolder (let's say http://www.mysite.com/subfolder/typo3) and CSS files of the backend compressed, the EXT:form/Resources/Public/Css/form.css is not found and the resulting compressed file is empty.

\TYPO3\CMS\Form\Controller\AbstractBackendController::resolveResourcePaths should return array(0 => 'typo3/sysext/form/Resources/Public/Css/form.css') but returns array(0 => '/subfolder/typo3/sysext/form/Resources/Public/Css/form.css')

This is passed to \TYPO3\CMS\Core\Page\PageRenderer::doConcatenateCss which passes the cssFiles list to \TYPO3\CMS\Core\Resource\ResourceCompressor::concatenateCssFiles. Here the file is not found and an empty compressed file is generated.

The rest of BE CSS files are rendered correctly. Forms css files are declared in EXT:/form/Configuration/Yaml/FormEditorSetup.yaml:10. As this is declared in a YAML file, maybe the CSS compression is using a different process which handles this differently.