WindowMargins is an option for Notebook which specifies what margins to leave around the window that is used to display the notebook on the screen.
specifies the distances from each edge of your screen to each edge of the window.
Typically only two distances are given explicitly; the others are Automatic, indicating that they should be determined from the size of the window.
Explicit distances are given in printer's points.
Negative values represent edges that are off the screen.
The settings for WindowMargins change whenever you move a window around interactively using the front end.
Window edges closer to the edges of the screen are typically assigned explicit margin distances; the others are set to Automatic. This allows the same setting for WindowMargins to work on screens of different sizes.
Automatic} all four margin distances must be given explicitly.
With the default setting WindowMargins->Automatic, new windows are placed on your screen in such a way as to make as many window title bars as possible visible.