qt form fullscreen sometimes goes over the ubuntu top pane and sometimes not - c++

Question

I am writing an QT application that embeds Mplayer on Ubuntu unity. I want to project videos on a projector in a dance show. I therefore have created a second form, which I move to the second screen and make it fullscreen.
Here is how I did it:
void MainWindow::on_testdialog()
{
QScreen *secondscreen = QApplication::screens()[1];
outputform->move(secondscreen->geometry().x(),secondscreen->geometry().y());
outputform->resize(secondscreen->geometry().width(),secondscreen->geometry().height());
outputform->setWindowFlags(Qt::Window | Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint);
outputform->setWindowState(Qt::WindowFullScreen);
outputform->show();
}
Then I redirect the Mplayer output to this form.
This works, however sometimes the second form is true fullscreen and sits on top of the Ubuntu unity menu pane (the one on top) and sometimes the Ubuntu unity menu pane is on top.
It seems to me that it is randomly doing this.
I would like my form to be always on top, because I don't want the menu pane to show up on stage.
Is there a way to do this?
Is there a reason for this, seamingly, random behavior? maybe a bug in Ubuntu?
Kind regards,
Bart.

Answer 1

This sounds like a problem of the window manager which makes it out of reach for Qt to solve. Qt is only setting the corresponding WM hints on the window and hopes for the window manager to do the right thing.
For your application I suggest you install a lightweight window manager like Openbox or Fluxbox that will more likely stay out of the way. After installation you can choose it in the login screen.