learn about Vue
- v-if: if it's false, it won't exist
- v-show: it's false, it will be hidden.
mouse event
<div class="box" @mouseover="handleEvent($event, 5)">mouseover(enter)</div>
<div class="box" @mouseleave="handleEvent">mouseleave</div>
<div class="box" @dbclick="handleEvent">double click</div>
<div class="box" @mousemove="handleMousemove">positon - {{x}} {{y}}</div>
