window decoration

At the moment, window decoration is a (mostly rectangular) frame surrounding another rectangular frame that holds the content of the window (let's say, the "content area"). I don't know how far this is technically conditioned - as far as I know the window decoration is painted by the window manager. Especially I don't know how far it would be possible to replace that approach by the one I would like to describe here.

I would like a window to be viewed as a surface on which the content may be arbitrarily placed. You might reply: what's the difference. The difference is, that at the moment the "content area" is a rectangular surface where content may be arbitrarily placed, but not the window as a whole. All the space within the "content area" is to be paintet by the application that owns the window, even space that isn't used. This is, for example, the space on the right side of a tool bar where no icons reside. If the window decoration would be a surface "behind" the content area, empty space would mean that the user can see this surface instead of the "empty window content", as is the background of the tool bar. Of course, there would be some space to be reserved for common tasks, like window title, min/max/close buttons and so on. Semi transparent content of the window would have the effect that the user looks through the content on the window surface. The look and feel of a tool bar (and other window content) could be better integrated with the look and feel of the window decoration by just painting the icons on that surface instead of painting the icons on a mostly grey surface that represents the background of the tool bar.

As always, that idea isn't originally from me. Please excuse me if I mixed up some technical facts, but I don't better know about it.