pinafore/routes/_components/virtualList/VirtualListHeader.html

<div class="virtual-list-header {{shown ? 'shown' : ''}} {{fadedIn ? 'faded-in' : ''}}"
     ref:node >
  <:Component {component} :virtualProps />
</div>
<style>
  .virtual-list-header {
    position: absolute;
    top: 0;
    width: 100%;
    opacity: 0;
    z-index: 10;
    transition: none;
    display: none;
  }
  .virtual-list-header.shown {
    display: block;
    transition: opacity 0.333s linear;
  }
  .virtual-list-header.faded-in {
    opacity: 1;
  }
</style>
<script>
  import { virtualListStore } from './virtualListStore'
  import { AsyncLayout } from '../../_utils/AsyncLayout'
  import { doubleRAF } from '../../_utils/doubleRAF'

  export default {
    oncreate() {
      this.observe('shown', shown => {
        if (shown) {
          this.doCalculateHeight()
          doubleRAF(() => this.set({fadedIn: true})) //  animate in
        } else {
          this.set({fadedIn: false})
        }
      }, {init: false})
    },
    store: () => virtualListStore,
    methods: {
      doCalculateHeight() {
        if (this.get('heightCalculated')) { // only need to calculate once, it never changes
          return
        }
        this.set({heightCalculated: true})
        const asyncLayout = new AsyncLayout(() => '__header__')
        asyncLayout.observe('__header__', this.refs.node, (rect) => {
          asyncLayout.disconnect()
          this.store.setForRealm({headerHeight: rect.height})
        })
      }
    }
  }
</script>