pinafore/routes/_components/virtualList/VirtualListItem.html

<div class="virtual-list-item {{shown ? 'shown' : ''}}"
     aria-hidden="{{!shown}}"
     virtual-list-key="{{key}}"
     ref:node
     style="transform: translateY({{offset}}px);" >
  <:Component {component}
              virtualProps="{{props}}"
              virtualIndex="{{index}}"
              virtualLength="{{$length}}"
              on:recalculateHeight="doRecalculateHeight()"/>
</div>
<style>
  .virtual-list-item {
    position: absolute;
    top: 0;
    opacity: 0;
    pointer-events: none;
    transition: opacity 0.333s linear;
  }
  .virtual-list-item.shown {
    opacity: 1;
    pointer-events: auto;
  }
</style>
<script>
  import { virtualListStore } from './virtualListStore'
  import { AsyncLayout } from '../../_utils/AsyncLayout'

  export default {
    oncreate() {
      let asyncLayout = new AsyncLayout(node => node.getAttribute('virtual-list-key'))
      let key = this.get('key')
      asyncLayout.observe(key, this.refs.node, (rect) => {
        asyncLayout.disconnect()
        // update all item heights in one batch for better perf
        this.store.batchUpdateForRealm('itemHeights', key, rect.height)
      })
    },
    store: () => virtualListStore,
    computed: {
      'shown': ($itemHeights, key) => $itemHeights[key] > 0
    },
    methods: {
      doRecalculateHeight() {
        // Recalculate immediately because this is done on-demand, e.g.
        // when clicking the "More" button on a spoiler.
        let rect = this.refs.node.getBoundingClientRect()
        let key = this.get('key')
        let itemHeights = this.store.get('itemHeights')
        itemHeights[key] = rect.height
        this.store.setForRealm({itemHeights})
      }
    }
  }
</script>