This is a minor issue in the grand scheme of things but would, nonetheless, make a big difference to the user experience.
When you have a modal dialog open in Process Studio (eg calculation properties), it prevents you from doing things elsewhere in the application. Moreover, unless, you understand the issue, it's not clear what's going on.
There is also a functional downside in that, for example, one calculation stage cannot easily be compared to another, across two processes, because of this restriction.
Solution: for optimum experience, the modal dialog should be modal in relation to its parent window only (ie Process Studio)