This happens only on Mac (not Windows), and was ok in 4.8.
The following sample illustrates the problem:
The splash screen stays on top of the window, and clicking on it does not help.
I am aware that in my example, "splash.finish" could be used to avoid the problem. But while launching an application, some unattended window might popup, out of our control (like plugin message, error message...), and in that case the splash screen might overlap the window and block everything from the user's point of view.

#include <QtWidgets>
#include <QtGui>
 
int main(int argc, char** argv)
{
    QApplication app(argc, argv);
    QPixmap pixmap(QSize(400, 300));
	pixmap.fill();
    QSplashScreen splash(pixmap);
    splash.show();
	QMessageBox::aboutQt(0);
    QMainWindow window;
    window.show();
    return app.exec();
}