semaphore/routes/_components/VirtualListItem.html

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

  const keyGetter = node => node.getAttribute('virtual-list-key')
  const asyncLayout = new AsyncLayout(keyGetter)

  export default {
    oncreate() {
      let key = this.get('key')
      asyncLayout.observe(key, this.refs.node, (rect) => {
        // update all item heights in one microtask batch for better perf
        this.store.batchUpdate('itemHeights', key, rect.height)
      })
    },
    ondestroy() {
      let key = this.get('key')
      asyncLayout.unobserve(key, this.refs.node)
    },
    store: () => virtualListStore,
    computed: {
      'shown': ($itemHeights, key) => $itemHeights[key] > 0
    },
    methods: {
      doRecalculateHeight() {
        let tempAsyncLayout = new AsyncLayout(keyGetter)
        let key = this.get('key')
        tempAsyncLayout.observe(key, this.refs.node, (rect) => {
          tempAsyncLayout.disconnect()
          // update all item heights in one microtask batch for better perf
          this.store.batchUpdate('itemHeights', key, rect.height)
        })
      }
    }
  }
</script>