textarea外层添加div，textarea相对外层div绝对定位，此时textarea的高度等于外层div的高度；
textarea同级添加div，同级div相对外层div相对定位，设置div的innerText等于textarea的value，那么当textarea输入内容增多引起换行时同级div也会换行， 同级div的高度会撑开外层div的高度。textarea的高度又会随着外层div的高度变化。以此实现textarea自适应。
visiblity:hidden; 隐藏同级div
设置同级div的 white-space: pre-wrap，识别enter换行
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.outter-div{
border: 1px solid red;
position: relative;
width: 100%;
}
.outter-div textarea{
border: 1px solid green;
position: absolute;
height: 100%;
width: 100%;
line-height: 16px;
}
.inner-div{
position: relative;
min-height: 16px;
white-space: pre-wrap;
line-height: 16px;
visibility: hidden;
}
</style>
</head>
<body>
<div id="app" class="outter-div">
<textarea v-model="text"></textarea>
<div class="inner-div">
{{ text }}
</div>
</div>
<script src="https://unpkg.com/vue@next"></script>
<script>
const app = Vue.createApp({
props: {},
setup(){
let text = Vue.ref('')
return {
text
}
}
})
app.mount("#app")
</script>
</body>
</html>
