downTolerance is ignored when scroll position is 0

Reduced test case:
https://codesandbox.io/s/vue-template-eowu8?fontsize=14

My expectation of the `downTolerance` prop would be it is consistently used regardless of scroll position.

If for some reason this doesn't make sense I feel like that the prop description should include that it is ignored if the page is already at the top of the scroll box.