.custom-combobox-menu { max-width:600px; 

          overflow-y: scroll;
          overflow-x: hidden;
          max-height: 400px;
 }

  .custom-combobox {
    position: relative;
    display: inline-block;
    padding-right:30px;
   
  }
  .custom-combobox-toggle {
    position: absolute;
    top: 0;
    bottom: 0;
    margin-left: -1px;
    padding: 0;
  }
    .custom-combobox-toggle::after {
        content: " ";
        display: inline-block;
        width: 12px;
        height: 12px;
    }
    .custom-combobox-input {
        margin: 0;
        padding: 5px 10px;
        width: 350px;
    }