I don't know what you mean by "pcx file", but I'd note that it's very likely each window does not have its "own scene tree", but they rather own a branch of the same scene tree:
- MainWindow <-- viewport
- SubWindow1 <-- viewport
- SubWindow2 <-- viewport
So it's almost the same than having those subwindows being
WindowDialogs like now, really. Almost nothing changes, except viewports can now render in other windows. This may also be quite relevant to the "emulated window system" feature, for platforms where native windows are not supported.