Click the button, it will work
watch for pop up above the button
to see that it did in fact click

You've now scrolled 300 px and
the button doesn't work anymore
try clicking it, it does nothing

However, click just above here
and it will work, watch popup
above button to see that it
did in fact work

Lastly, scroll the page a tiny bit with your finger and the button will start to work again how it should.