This is Pantheon's recommended starting point for forking new Drupal upstreams that work with the Platform's Integrated Composer build process. It is also the Platform's standard Drupal 9 upstream.
Unlike with earlier Pantheon upstreams, files such as Drupal Core that you are unlikely to adjust while building sites are not in the main branch of the repository. Instead, they are referenced as dependencies that are installed by Composer.
For more information and detailed installation guides, please visit the Integrated Composer Pantheon documentation: https://pantheon.io/docs/integrated-composer
Contributions are welcome in the form of GitHub pull requests. However, the
pantheon-upstreams/drupal-composer-managed repository is a mirror that does not
directly accept pull requests.
Instead, to propose a change, please fork pantheon-systems/drupal-composer-managed and submit a PR to that repository.