But that's not a problem. If the user closes the browser or tab, the attack can continue at a later point in time. It doesn't need to be collected all at once. As a concrete example, if you leave your computer at work running during the weekend, the attacker has enough time. Or if you leave it running during the night, the attack can be spread out over a few nights. I'm sure there are even more scenarios than just these two examples: there's room for quite some flexibility when performing the attack.