overflow生效的前提：DOM元素宽高是个已知的常量方法一：父元素flex，兄弟元素height固定，自身flex:

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .parent {
      width: 500px;
      height: 800px;
      background-color: pink;

      /* 方法一 */
      /* display: flex; */
      /* flex-direction: column; */
    }
    .first {
      height: 48px;
      background-color: yellow;
    }
    .second {
      /* 方法一 */
      /* flex: 1; */

      /* 方法二 */
      height: calc(100% - 48px);

      background-color: green;
      overflow-y: auto;
    }
    .item {
      height: 300px;
      background-color: purple;
    }
  </style>
</head>

<body>
  <div class="parent">
    <div class="first"></div>
    <div class="second">
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
    </div>
  </div>
</body>

</html>

方法一：父元素flex，兄弟元素height固定，自身flex:1 方法二：自身height = calc(父元素height - 所有兄弟元素height)