I encountered this problem while browsing through docs at https://training.shoppinpal.com/feedback.html and https://training.shoppinpal.com/loopback/Promises-191e.html
I encountered it twice on different topics but upon page refresh, it did not occur. I think it is happening randomly! As you can see I get a pop up from browser!

Browser: Google chrome ver 56.0.2924.87
OS: Android 6.0.1
Smartphone: One Plus One (A0001)