Method to know when 3D content actually appears on screen

When trying to improve startup times, we are most interested in the time from launch to when 3D content is actually visible on the screen. There does not seem to be a programmatic way to know this, however. FrameAction, for instance, can be triggered up to 50 times before 3D content actually appears on the screen. Also, if 3D content is animated too soon, issues seem to arise, plus you probably wouldn't want to start animating before it's visible anyway, so knowing when it's visible is useful for that too.