resize() after show moves widget to (0,0), putting frameheader out of screen

Resizing a widget, which is not child of any other widget, causes the widget to be resized but also to be moved to (0,0) on screen, putting the header and left border out of screen. The following example demonstrates it:
#include <QApplication>
#include <QWidget>
#include <QMainWindow>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
// QWidget w;
QMainWindow w;
w.show();

w.resize(200, 200);

return a.exec();
}
That occurs with any kind of widget (switch the type of w to see).
This appears to be a regression, as this kind of behaviour is not shown using Qt 4.8.x