A quick and final update: https://bugreports.qt.io/browse/QTBUG-72641 The topic was reopened - in no small part due to @Kalileo I believe ;-) - and the error seems to be found and fixed. The fixed version seems to be the upcoming 5.12.1 and onwards. Thanks everyone for the interest and help in the issue.

Hi and welcome to devnet, Please take a look at the Qt Quick Scene Graph chapter of Qt's documentation. You have there the explanation on how it's working and what you can use these signals for. There's also the Scene Graph - OpenGL Under QML example that shows how to use beforeRendering. Hope it helps

@p3c0 Thanks for your help, I will try your option, I hope I could use QQuickWidget instead of QQmlApplicationEngine with no problem or unexpected effect.

Hi Colin, I'm working on something similar. Was this issue ever resolved? If so, do you mind sharing? Thanks, Vishal

@Wieland said: I'm pretty sure there is no ootb solution to do that. Why do you want to render the background when it's not possible? Maybe we can find another solution. This is because I have a real-time video streaming and the frames are rendering in main QQuickWindow context. I think otherwise I need to create another opengl context and run rendering in it. Or maybe there is some way to use the same QQuickWindow context to do that?

And also http://doc.qt.io/qt-5/threads-qobject.html#qobject-reentrancy