Why does iOS Safari fire window focus event twice?

Question

I have the following code on a page:

var i = 0;
window.addEventListener("focus", function(event) { 
  document.body.innerHTML = "Focus count = " + i++;
}, false);

On desktop this increments correctly (1,2,3 etc) when switching tabs (or any window blurring/focusing action)

However, when I switch tabs on ios safari the event appears to be firing twice (1,3,5,7 etc)

Why is this happening?

Answer 1

ATTOW the reason is unknown, but this is a known issue in Webkit:

https://bugs.webkit.org/show_bug.cgi?id=179990

From above:

With two or more text inputs on a form, switching focus by click from one to another sometimes causes an extra focus/blur to fire.