SOURCE

<div class="wrapper">
  <div class="dropdown-flex">
    <div class="custom-dropdown-content-header">
         <div class="large-option-item">第三层div</div>
    <div class="flex-item2">第三层div</div>
    </div>
   
    <div class="dropdown-content-large">
      <div class="large-option-item">第三层div1111111111111111111111111111111111111111111111</div>
      <div class="large-option-item">第三层div222222222222222222222222222222222222</div>
    </div>
  </div>
</div>

.wrapper{
   Width:400px;
   overflow: 'auto';
}
.ant-select-dropdown {
  overflow: auto;
}

.dropdown-flex {
  display: flex;
  flex-direction: column;
  overflow: auto;
  max-width: 400px;
}

.custom-dropdown-content-header {
  max-height: 200px;
  display: flex;
  flex-direction: row;
  overflow: auto;
}

.dropdown-content-large {
  display: flex;
  flex-direction: row;
  justify-content: "space-around";
  align-items: "center";
  align-content: "center";
  white-space: "nowrap";
  width: 500px;
}
.large-option-item {

}