Go to definition (F12 in Visual Studio) is such an intensively used command, but it opens a new window for the document. Visual Studio 2013 adds a new go to definition command that actually displays the definition inline, in the same window. It’s called Peek Definition and the shortcut is ALT+F12.
It opens an embedded window inside the current window, displaying the definition.
However, it is possible to peek another definition and so on. All peeked definitions are displayed in the same window in a tabbed manner (the little circles on top are the tabs).
It is possible to re-size this window’s height and promote a particular tab to a new document window.