The fullscreen is set by calling the showFullScreen() method of QQuickView class which is inherited from QWindow (http://qt-project.org/doc/qt-5.0/qtg...showFullScreen).
A minimal example of code :
int main(int argc, char *argv)
QGuiApplication app(argc, argv);
viewer.setSource("... a qml file ...");
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When the app loose the focus (for example when doing Control + Escape) and it get the focus back after, the app doesn't repaint itself. The render is not updated anymore.