The X Window System, Version 11, is the standard window system on Linux and UNIX systems. X11, designed in 1987, was “state of the art ” at that time. From its inception, X has been a network transparent window system in which X client applications can run on any machine in a network using an X server running on any display. While there have been some significant extensions to X over its history (e.g. OpenGL support), X’s design lay fallow over much of the 1990’s. With the increasing interest in open source systems, it was no longer sufficient for modern applications and a significant overhaul is now well underway. This paper describes revisions to the architecture of the window system used in a growing fraction of desktops and embedded systems 1

Typical window management systems rely on direct manipulation techniques to organize and layout windows. Direct

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A constraint is a relation that we would like to maintain. Over the last thirty years, a number of problems have prevented constraints from being widely accepted for use in interactive graphical applications. The biggest difficulty of applying constraints has been finding the right tradeoff between performance and expressiveness. To be able to satisfy systems of constraints efficiently enough for real-time use, interactive applications have restricted the problem to solving less-expressive sets of constraints. One common such restriction is to require that the constraint relationships are acyclic, thus enabling simple techniques based on local propagation. Unfortunately, these limitations in expressiveness counteract the benefits that the declarative specification of relationships with constraints is inten...