It's a Qt-extension widget set for MDI that is an alternative to QWorkspace. Compared to QWorkspace, your application is configurable in several modes - MDI like QWorkspace and Unix-like with all windows as toplevel. You can compile QextMDI as a KDE2 extension library, it then bases on KMainWindow and kdeui.so. You can decorate the MDI views with Win9x, KDE1 and default-KDE2 windowmanager style. Take a look here for further info (including screenshots).