如何使用javascript强制下拉列表表现得像列表框

<select id="dropdown" onchange="updateTextbox()">
  <option value="option1">Option 1</option>
  <option value="option2">Option 2</option>
  <option value="option3">Option 3</option>
</select>
<input type="text" id="textbox" readonly>

#dropdown {
  display: none;
}

#textbox {
  border: 1px solid #ccc;
  padding: 5px;
  background-color: #fff;
  cursor: pointer;
}

function updateTextbox() {
  var dropdown = document.getElementById("dropdown");
  var textbox = document.getElementById("textbox");
  var selectedOption = dropdown.options[dropdown.selectedIndex].text;
  textbox.value = selectedOption;
}

function toggleDropdown() {
  var dropdown = document.getElementById("dropdown");
  dropdown.style.display = (dropdown.style.display === "none") ? "block" : "none";
}

window.onload = function() {
  var textbox = document.getElementById("textbox");
  textbox.onclick = toggleDropdown;
}