The add-on debugger only shows globals from the add-on you're debugging. Sometimes though code execution will flow into something else, a platform piece of JS for example. If you step into a piece of code like this the UI behaves strangely. I think the locals and variables display right but the code doesn't display. We should do something sane here, either open the code and maybe include it in the list of globals from then on, or just step completely over code that isn't part of the add-on. I think the former is preferable since I suspect we may find a bunch of cases where we don't list everything we need to at first.