pinafore/routes/_components/virtualList/VirtualListHeader.html

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

  export default {
    oncreate() {
      const asyncLayout = new AsyncLayout(() => '__header__')
      asyncLayout.observe('__header__', this.refs.node, (rect) => {
        asyncLayout.disconnect()
        this.store.setForRealm({headerHeight: rect.height})
      })
    },
    store: () => virtualListStore,
  }
</script>