0.) Set setHorizontalScrollBarPolicy(Qt::ScrollBarAsNeeded) and setVerticalScrollBarPolicy(Qt::ScrollBarAsNeeded)
1.) I have created QSubWindows in QMdiArea.
2.) Maximize MainWindow
3.) Execute tileSubWindows() method of QMdiArea object

Here the scroll bars are not loaded if the size of sub windows are brigger than QMidArea size. I have to minimize and maximize the MainWindow or move windows to activate scroll bars again.