Drupal on Pantheon git repo updating and site build best practices

The build process on Pantheon is done with Composer, the build prioritizes the composer.json and composer.lock file in the root of the repo, and builds out based on those two files. 

The Pantheon build process can balk anywhere it encounters files that it 'expects' to be from Composer being committed in the git repo itself.  Changing and committing files in directories where Composer puts files, especially force commits with git -f force to directories such as web/modules/composer can cause problems.

If you have custom code for themes or modules, they should be added to the directories below:

/web/sites/default/modules/custom

/web/sites/default/themes/custom

/web/sites/default/libraries

Pantheon also does not change its build restrictions based on changes to the .gitignore file.  Removing lines from the .gitignore and committing files in those now tracked directories can still lead to the Pantheon build process issues..

We therefore recommend committing only changes made with Composer to the composer.json and composer.lock files as the best practice for managing sites on Pantheon.  Libraries as well as modules can be added via Composer, and custom code should be added only to the appropriate /web/sites/default folder.

Resources:

What is Composer

Using Composer to install Drupal and manage dependencies

Updating a module