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Third button that closes current app might be accidentally tapped.
Maybe it's better to close app if it was long tap? And if it's short tap, use it as back button.
Also this feature might be used in Angelfish to make navigation easier. And address bar dragging in angelfish might be used to switch tabs instead of page navigation.
Reasons to implement this:

1. It's useful to have back button in different apps, like Angelfish
2. Close button might be tapped accidentally
3. Similar feature is already used to hide keyboard