code: function watchAppearForScrollables (tagName, context) {
    // when this is a scroller/list/waterfall
    if (scrollableTypes.indexOf(tagName) > -1) {
      const sd = context.scrollDirection
      if (!sd || sd !== 'horizontal') {
        appearWatched = context
        watchAppear(context, true)
      }
    }
  }
docs: if it's a scrollable tag, then watch appear events for it.