Function drain_frame_requests 

pub fn drain_frame_requests(
    events: MessageReader<'_, '_, ScriptCommandEvent>,
    hook: ResMut<'_, FrameHook>,
    animations: Option<Res<'_, AnimationsActive>>,
    waker: Option<Res<'_, EventLoopWaker>>,
)

Read the RequestFrame commands emitted this tick and arm the hook.

Registered after every set that calls into a host, because a request is only worth anything on the tick it was made: arming is what raises the animation flag and wakes the loop, so a request this system ran past does not merely arrive late, it lets an idle app park and never arrive at all. A request made from inside on_frame therefore arms the next tick rather than re-entering this one, which is what keeps a loop at one callback per tick.

Both the animation flag and the waker are optional so a bare test can drive these systems without a window.