DOMContentLoaded listener in popup-frame-script.js

[janodvarko]

DOMContentLoaded listener in popup-frame-script.js needs to check that the window has the expected URL, or at least to unregister itself after the first response, so that it doesn't wind up injecting a privileged function into a remote document if the window somehow navigates somewhere unexpected.

(from AMO review)

Honza

[janodvarko]

@aliuk2012 This came from the AMO reviewer, would you be interested to take a look?

Honza