Tip of the day: how to debug Composer scripts with XDebug and PhpStorm

The problem: XDebug doesn't work for Composer scripts

PhpStorm is quite convenient to debug scripts with XDebug (do you support Derick for giving us XDebug ?): just add a "Run/Debug configuration", choosing the "PHP Script" type, give a few parameters, and you can start debugging your PHP CLI scripts, using breakpoints, evaluations, etc.

Wonderful. So now, let's define such a configuration to debug a Composer script, say a Behat configuration generator from site settings for some current Drupal 8 project. Apply the configuration, run it in debug mode, and ....

...PhpStorm doesn't stop, the script runs and ends, and all breakpoints were ignored. How to actually use breakpoints in the IDE ?