Hi there,
NextGen is popular, but it’s a nightmare in my opinion. You just need to view your error_log file with it active to see the mass amount of errors and warnings it generates.
The back to top button just uses a handful of javascript lines to work – nothing fancy or complicated.
I’m not sure why NextGen stops the JS from firing – there’s not even any javascript errors in the console to give me any hints.
I’ll try to find some time to play with it, but it seems like kind of a lost cause when I see how many errors the plugin generates, regardless of the popularity of it.