Described is a pluggable policy component that determines the look and feel, or windows visual experience, of a computer user interface. Window-related instructions are redirected to the policy component, while client area change instructions are provided to a substrate (into which the policy component plugs in) that includes a composition component. The plug-in policy component handles windows-related (e.g., structural or attribute) changes to a scene graph to construct and maintain the scene graph, while the substrate handles program content changes to the client areas within windows. The substrate may include a desktop window manager that has access to the client areas in the scene graph, whereby the desktop window manager can copy a client area for rendering. For example, the desktop window manager can provide a supplemental live thumbnail image of a window.