Monorepo: Composer folder settings only available in some popups?

I have a monorepo setup with two webapp folders and a few package folders (Symfony bundles).

There are a couple of situations where a composer settings popup appears with a list of folders and composer.json files.

However when I go to the main composer settings this list is not available. I am hoping to configure the project to understand more than one composer root folder. 

I understand from other posts on here that PHPStorm doesn't work with monorepos so maybe it doesn't matter. Still, is there a way to access this list other than when it pops up?

This has a list of folders:

 

This does not: