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CURRENT DEFAULT "BACKSPACE" BEHAVIOR IN MANY BROWSERS:
(this example is not prescriptive, it's just what happens)
At any rate, the GIF shows the current situation. You should watch it.
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MY COMMENTS:
I actually wrote to this app creator that they should throw up a confirmation window ( like these https://www.google.com/search?q=confirm+navigation&tbm=isch )
but the fact is that the browser is the one that decided to navigate away. Now what's very interesting, is that even in this, HN's, thread we have people saying "Yes!!!" and people saying "No!!!" to the change.
So people who simply have never used backspace for navigation, like me, have many times accidentally touched backspace or thought we were focused on a form, and ended up losing data (because the page didn't throw up a confirmation window after navigating back, and after clicking forward the page is blank again.) While other people, who have no convenient single key they can use to navigate back, have come to rely on it. I'm not sure what the solution is, but here's the current situation so everyone understands it.