Function stepped_frame 

pub(crate) fn stepped_frame(
    worlds: SceneHandles,
    stepper: SceneStepper,
) -> OnFrame

Step the scene’s worlds once per frame, before the draw.

A stepped run has no threads, so the host steps what a threaded build would leave to them. The stepper follows the frame’s own duration and carries the step remainders, so the worlds keep their rates whatever the display redraws at.