The icon below shows at the top of the vertical scrollbar in Visual Studio 2012’s text editor. When I drag it down, another page shows behind the current visible page. It looks like a copy of it. (I am using dark theme)
What is it used for? I don’t know what the feature is called, therefore I couldn’t search for it. I am not sure if it’s native to VS 2012 or it’s from one of the VS add-ons I am using.
Sometimes the icon disappears. Why?

It’s for split viewing of a file. So you can view part of the file and edit another part.
It’s a common feature in advanced text editors.