How to tell if the return vertex icon is displayed

How to tell if the return vertex icon is displayed
I want to add a return vertex to the page if the page is fully displayed, the icon that returns the vertex does not appear
If the page can be slid down to show the icon of the returned vertex how do you decide?
Thank you
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JS determines whether the scroll bar appears or determines the position of the screen and then controls whether the div shows hidden.
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Http://www.cnblogs.com/jingangel/archive/2012/03/08/2385939.html
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