First I create a QMdiSubWindow In QMdiArea, and set it as a native window, the codes may be look like those:

QMdiArea *pMdiArea = new QMdiArea();
layout->AddWidget(pMdiArea);
QMdiSubWindow *pWindow = new QMdiSubWindow();
pWindow->setAttribute(Qt::WA_NativeWindow);
pMidArea->AddSubWindow(pWindow);
pWindow->showMaximized();

Second, I got the HANDLE of QMdiSubWindow by spy++ and run the follow Windows API code:

SetWindowPos(handle, NULL, 0, 0, 200, 200, 0x2);

The Size of QMdiSubWindow did not changed.But the size got by Windows API (GetWindowPos)has been changed!