Function retire_frame_request 

pub fn retire_frame_request(
    hook: ResMut<'_, FrameHook>,
    tick: Res<'_, Tick>,
    due: ResMut<'_, DueFrame>,
)

Decide whether a frame callback is due this tick and what step it gets.

Host-neutral and registered once, so every active host is offered the same frame instead of the first one to run taking it. The request is consumed here, before any host fires, so a callback that asks for another is asking for the next tick and cannot re-enter this one.

The first frame of a loop gets 0.0: no callback ran before it, so there is no interval to report and a made-up one would move an animation before any time passed. Every frame after that gets the tick’s own elapsed time, capped at MAX_FRAME_DT.