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You use the onbeforeunload event, which doesn't block the event thread. (EDIT: actually it does, just not while JS code is executing, unlike alert().)

In fact, the HTML5 spec permits browsers to disregard the normal blocking behavior of alert()/confirm()/prompt() (making them no-ops) while this event is being handled: https://developer.mozilla.org/en-US/docs/DOM/window.onbefore... the dialog box is open

I managed to break that link somehow and can no longer edit it, here's the correct one: https://developer.mozilla.org/en-US/docs/DOM/window.onbefore...

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