Function contentStream

({
  $isShown,
  $periodicCheck,
  $triggerContentChange,
  article,
  scroller,
  addDisposer,
}: {
  article: HTMLElement
  scroller: HTMLElement
  $triggerContentChange: Stream<null>
  $isShown: Stream<boolean>
  $periodicCheck: Stream<null>
  addDisposer: (unsub: () => void) => number
})