scrollMixin.js

const scrollMixin = {
    data() {
        return {
            pageOffset: 0
        }
    },
    mounted() {
        window.addEventListener('scroll', this.update)
    },
    destroyed() {
        window.removeEventListener('scroll', this.update)
    },
    methods: {
        update() {
            this.pageOffset = window.pageYOffset
        }
    }
}