WindowClickSelect

WindowClickSelect is an option for Notebook which specifies whether the window for the notebook should become selected if you click on it.

With WindowClickSelect->True, clicking on the window corresponding to a notebook makes that notebook the currently selected one.

WindowClickSelect affects selection of a window as a whole; Selectable affects only selection of the contents of a window.

See The Mathematica Book: Section 2.10.13.

See also: WindowFloating, SetSelectedNotebook.