Up/down button

Create a "to the top" button to help with page scrolling.

It should work like this:

While the page is not scrolled down at least for the window height -- it's invisible.
When the page is scrolled down more than the window height -- there appears an "upwards" arrow in the left-top corner. If the page is scrolled back, it disappears.
When the arrow is clicked, the page scrolls to the top.

Like this (top-left corner, scroll to see):

[iframe border="1" height="200" link src="solution"]