You're allowed to use the code for whatever else you like, though. For instance, if you want to grab some of their HTTP-handling code, or maybe the markup parser, or perhaps look at how they got modeless key combinations to work in the browser… you can just copy-and-paste into any GPLv3 or AGPLv3 codebase, add a comment for attribution and you're done.
GPL is good for more than just forking.