form.css compression in BE not working when TYPO3 is in a subfolder
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.