Qt FullScreen on Startup

I already faced this problem and a very nice solution was to delay the fullscreen switch by one second (using a QTimer):

QTimer::singleShot(0, this, SLOT(showFullScreen()));

use the following if you want to have the app open as maximized window:

Mainwindow w;
w.setWindowState(Qt::WindowMaximized);
w.show();

use the following if you want to have the app open as fullscreen window:

Mainwindow w;
w.setWindowState(Qt::WindowFullScreen);
w.show();