Create a trigger to know when 3D scene really drawing on the screen

Currently, on starting Cluster all cameras are disabled. When Cluster starts and the first Q3DSSurfaceViewer::frameUpdate arrives Cluster asks Animation Player to allow to enable cameras on the 3D scene. If it is allowed, cameras are switched on. After that Cluster should notify Animation Player that Cluster actually draws something on the screen and Animation Player can stop rendering it-self content.
The problem is Cluster cannot know when some of its content starts drawing on the screen actually. Experiments show that the first and second frames are transparent. Looks like the third frame usually has content. However, it seems not a good solution to use the third frame as a trigger for synchronization with Animation Player.
Could you suggest a solution to how Cluster can know when content actually starts being drawn?