Function transient 

pub fn transient(
    id: impl Into<SharedString>,
    handle: &ScrollHandle,
    state: &TransientState,
    reduce_motion: bool,
) -> AnyElement

The same bar as scrollbar, but it only earns its place while the content moves: activity raises the thumb, and it fades out over TRANSIENT_IDLE once the scrolling stops. Hovering the strip or dragging the thumb holds it up. With reduce_motion there is nothing to animate, so it renders as the always-on bar.