setFixedSize doesn't work as expected

Consider the following code (MainWindow derived from QMainWindow):

MainWindow::MainWindow()
{
    setFixedSize(256, 59);
    show();
}

If I run this with Qt 4.8 - I get a normal non-resizeable window.

However, if I run this with Qt 5.1 - I can still see double-sided arrows, when I hover my mouse over the frame and I can resize the window vertically by 14 pixels. If I put 73 instead of 59 - it becomes truly fixed, but the arrows do not disappear. If I put setWindowFlags(Qt::MSWindowsFixedSizeDialogHint) between setFixedSize and show - works as expected.