Hmmm, yeah i can't really figure out how these modals are being shown. It appears to not be using the normal bootstrap way of displaying modals, but just (kind-of) mis-using the CSS for the modals.

Tried to figure out how they are being displayed in the react code, but to no avail. :(

Maybe I can help with that, since the misuse comes from me ;-)
It's all in hubs/static/client/app/components/Modal.js. Since we can't use Bootstrap's Jquery I thought I would just add the right elements with the right css classes and it would just work, right? Wrong. :-D

OKies! i figured it out!

bootstrap js adds an additional class to the .modal-open that it uses to control the scrolling. add that class to the body when the modal is open, and it all works as expected.

Problem is, i have no idea how to do that from within react :/

Commit 96ae1437 fixes this issue