如果不需要动画，可以设置 max-height 到 fit-content 否则，您可以添加元素的 scrollHeight 到它的父母的高度。为了避免设置太大的数字（如果同一子项打开/关闭了多次），您可以添加 transitionend 即使是听者也要设置正确的 max-height 家长的姓名：

var coll = document.getElementsByClassName("collapsible");
var i;

for (i = 0; i < coll.length; i++) {
  coll[i].addEventListener("click", function() {
    this.classList.toggle("active");
// managed by css
    var content = this.nextElementSibling;
    if (content.style.maxHeight){
      content.style.maxHeight = null;
    } else {
      content.style.maxHeight = content.scrollHeight + "px";
      content.parentNode.style.maxHeight = (parseInt(content.parentNode.style.maxHeight) + content.scrollHeight) + "px";
    } 
  });
  coll[i].nextElementSibling.addEventListener("transitionend", function()
  {
      this.parentNode.style.maxHeight = this.parentNode.scrollHeight + "px";
  });
}

.collapsible {
  background-color: #2d2e2e;
  color: white;
  cursor: pointer;
  padding: 10px;
  width: 100%;
  border: none;
  text-align: left;
  outline: none;
  font-size: 15px;
}

.active, .collapsible:hover {
  background-color: black;
}

.collapsible:after {
  content: '\002B';
  color: white;
  font-weight: bold;
  float: left;
  margin-right: 5px;
}

.active:after {
  content: "\2212";
}

.collapse_content {
  padding: 0 18px;
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.1s ease-out;
}

<button class="collapsible">Full Collapsible Set:</button>
<div class="collapse_content">
  <button class="collapsible">Open Section 1</button>
  <div class="collapse_content">
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
  </div>
  <button class="collapsible">Open Section 2</button>
  <div class="collapse_content">
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco.</p>
  </div>
  <button class="collapsible">Open Section 3</button>
  <div class="collapse_content">
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
  </div>
</div>