Apple Patent: Translucency
On November 5, 2003, Apple filed a patent for an interesting user interface technique entitled "Graduated visual and manipulative translucency for windows".
The interface is described as follows:
Methods and systems for providing graphical user interfaces are described. overlaid, Information-bearing windows whose contents remain unchanged for a predetermined period of time become translucent. The translucency can be graduated so that, over time, if the window's contents remain unchanged, the window becomes more translucent. In addition to visual translucency, windows according to the present invention also have a manipulative translucent quality. Upon reaching a certain level of visual translucency, user input in the region of the window is interpreted as an operation on the underlying objects rather than the contents of the overlaying window.
Such a method would give active windows more visual importance on the user's screen, while unused windows would slowly fade into the background.