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):

샌드박스를 열어 정답을 작성해보세요.