UP button to scroll up from long lists

[estellecomment]

From spec : https://docs.google.com/document/d/1UwSe8H3xykvOUwu65KxJYgBTxv40Y_9b3CPgRbcXOH0/edit#bookmark=id.ta51n5b830h

The UP button is stuck to the bottom of the viewport. When does the UP button appear?

• If there is a Tasks list, when you scroll far enough that the Tasks header is hidden from view
• If there is no Tasks list, same with the History header.
• if there's neither Tasks or History, then no UP button at all.

When you click the UP button, you go back to the top of the page (not the top of the Tasks or History list, because we can't know which one you'd want to go to. This is a page scroll).

[SCdF]

Also, worth thinking about what happens when the button covers content you'd expect to be able to read. Need to make sure if the button is visible you can scroll down enough so the button is only over whitespace and not content. See the Gmail Android app as an example of how to get this right.

[sglangevin]

Zepelin mocks -- see comments: https://zpl.io/2tsIvg. This is a duplicate of #2894 but I will close that one in favor of this one to preserve the discussion here.

[estellecomment]

(refer to design spec for full description : https://docs.google.com/document/d/1UwSe8H3xykvOUwu65KxJYgBTxv40Y_9b3CPgRbcXOH0/edit#bookmark=id.ta51n5b830h)