this.$nextTick(() => {
                this.tagOverflow = this.$refs.ul.offsetWidth > this.$refs.outer.offsetWidth;
                if (!this.tagOverflow) {
                    this.tagBodyLeft = 0;
                }
            });

:class="{'tags-overflow':tagOverflow}"

.tags .tags-outer {
    position: absolute;
    left: 0;
    right: 120px;
    top: 0;
    bottom: 0;
}
.tags-overflow .tags-outer {
    left: 30px;
    right: 135px;
}
.tags-btn {
    display: none;
    position: absolute;
    width: 25px;
    height: 30px;
    text-align: center;
    line-height: 30px;
    cursor: pointer;
    background-color: #fff;
    z-index: 10;
}
.tags-overflow .tags-btn {
    display: block;
}