2019 jetbrains publishes a howto to be able to step-debug twig templates. Yes! The user-edited ones. As well as the compiled-to-php ones.
Lucky me stumbled over it when I needed exactly that functionality right now!
Thought I'd share it: https://blog.jetbrains.com/phpstorm/2019/05/twig-and-blade-templates-debugging-2/
@heiglandreas now I need to test this ;)