scrollSetupComponent.vue

// ...
<script>
import { value, onMounted, onUnmounted } from 'vue-function-api'

export default {
  setup(props) {
    const pageOffset = value(0)
    const update = () => {
        pageOffset.value = window.pageYOffset
    }
    
    onMounted(() => window.addEventListener('scroll', update))
    onUnmounted(() => window.removeEventListener('scroll', update))
    
    return {
      pageOffset
    }
  }
}
</script>
// ...