Starting from Visual Studio 2013, the Peek Definition feature is available in the editor. It allows you to see source code of a referenced symbol without opening a new tab. With ReSharper, this feature works a little bit different:
All navigation features are available in the peek definition view. For example, you can invoke the Navigate To command (Ctrl+Shift+G). All navigation results are shown within the same peek definition view.
All refactorings are available in the peek definition view. You can press Ctrl+Shift+R to check the list of available refactorings.
You can close the definition widow by pressing Esc and switch between the editor and the definition window by pressing Shift+Esc.