useScrollSave(enabled: boolean,onSave: (scrollY: number) => void): void
Calls onSave(scrollY) when the component unmounts or the tab is hidden
(pagehide), while enabled. Feed pages pass enabled once their feed is
loaded, and an onSave that calls useFeedCache's saveState.
It doesn't save on scroll, since serializing the whole feed there took a long task at every pause.