Not sure what to say here -- in exploring more tabs, it's not always happening, but is doing so much of the time for me. (Including on this page.)
Interesting: in exploring more, it's intermittent -- in the past ten minutes, I've observed at least a couple of pages where it succeeded the first time, but failed subsequently. (Eg, that just happened for a couple of GDocs I have open.) Possibly it's a state that the browser is getting into, although there's no obvious cause -- after a couple of successful reloads, now the button is consistently unresponsive for me again.