function growDiv() {
  var growDiv = document.getElementById('grow');
  if (growDiv.clientHeight) {
    growDiv.style.height = 0;
  } else {
    var wrapper = document.querySelector('.measuringWrapper');
    growDiv.style.height = wrapper.clientHeight + "px";
  }
}

#grow {
  -moz-transition: height .5s;
  -ms-transition: height .5s;
  -o-transition: height .5s;
  -webkit-transition: height .5s;
  transition: height .5s;
  height: 0;
  overflow: hidden;
  outline: 1px solid red;
}

<input type="button" onclick="growDiv()" value="grow">
<div id='grow'>
  <div class='measuringWrapper'>
    <div>
      The contents of my div.
    </div>
    <div>
      The contents of my div.
    </div>
    <div>
      The contents of my div.
    </div>
    <div>
      The contents of my div.
    </div>
    <div>
      The contents of my div.
    </div>
    <div>
      The contents of my div.
    </div>
  </div>
</div>

function growDiv() {
  var growDiv = document.getElementById('grow');
  if (growDiv.clientHeight) {
    growDiv.style.height = 0;
  } else {
    growDiv.style.height = 'auto';
  }
}

#grow {
  -moz-transition: height .5s;
  -ms-transition: height .5s;
  -o-transition: height .5s;
  -webkit-transition: height .5s;
  transition: height .5s;
  height: 0;
  overflow: hidden;
  outline: 1px solid red;
}

<input type="button" onclick="growDiv()" value="grow">
<div id='grow'>
  <div>
    The contents of my div.
  </div>
  <div>
    The contents of my div.
  </div>
  <div>
    The contents of my div.
  </div>
  <div>
    The contents of my div.
  </div>
  <div>
    The contents of my div.
  </div>
  <div>
    The contents of my div.
  </div>
</div>

#menu #list {
    max-height: 0;
    transition: max-height 0.15s ease-out;
    overflow: hidden;
    background: #d5d5d5;
}

#menu:hover #list {
    max-height: 500px;
    transition: max-height 0.25s ease-in;
}

<div id="menu">
    <a>hover me</a>
    <ul id="list">
        <!-- Create a bunch, or not a bunch, of li's to see the timing. -->
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
    </ul>
</div>

var content = $('#content');
content.inner = $('#content .inner'); // inner div needed to get size of content when closed

// css transition callback
content.on('transitionEnd webkitTransitionEnd transitionend oTransitionEnd msTransitionEnd', function(e){
    if(content.hasClass('open')){
        content.css('max-height', 9999); // try setting this to 'none'... I dare you!
    }
});

$('#toggle').on('click', function(e){
    content.toggleClass('open closed');
    content.contentHeight = content.outerHeight();
    
    if(content.hasClass('closed')){
        
        // disable transitions & set max-height to content height
        content.removeClass('transitions').css('max-height', content.contentHeight);
        setTimeout(function(){
            
            // enable & start transition
            content.addClass('transitions').css({
                'max-height': 0,
                'opacity': 0
            });
            
        }, 10); // 10ms timeout is the secret ingredient for disabling/enabling transitions
        // chrome only needs 1ms but FF needs ~10ms or it chokes on the first animation for some reason
        
    }else if(content.hasClass('open')){  
        
        content.contentHeight += content.inner.outerHeight(); // if closed, add inner height to content height
        content.css({
            'max-height': content.contentHeight,
            'opacity': 1
        });
        
    }
});

.transitions {
    transition: all 0.5s ease-in-out;
    -webkit-transition: all 0.5s ease-in-out;
    -moz-transition: all 0.5s ease-in-out;
}

body {
    font-family:Arial;
    line-height: 3ex;
}
code {
    display: inline-block;
    background: #fafafa;
    padding: 0 1ex;
}
#toggle {
    display:block;
    padding:10px;
    margin:10px auto;
    text-align:center;
    width:30ex;
}
#content {
    overflow:hidden;
    margin:10px;
    border:1px solid #666;
    background:#efefef;
    opacity:1;
}
#content .inner {
    padding:10px;
    overflow:auto;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<div id="content" class="open">
    <div class="inner">
        <h3>Smooth CSS Transitions Between <code>height: 0</code> and <code>height: auto</code></h3>
        <p>A clever workaround is to use <code>max-height</code> instead of <code>height</code>, and set it to something bigger than your content. Problem is the browser uses this value to calculate transition duration. So if you set it to <code>max-height: 1000px</code> but the content is only 100px high, the animation will be 10x too fast.</p>
        <p>Another option is to measure the content height with JS and transition to that fixed value, but then you have to keep track of the content and manually resize it if it changes.</p>
        <p>This solution is a hybrid of the two - transition to the measured content height, then set it to <code>max-height: 9999px</code> after the transition for fluid content sizing.</p>
    </div>
</div>

<br />

<button id="toggle">Challenge Accepted!</button>

ul {
  background-color: #eee;
  transform: scaleY(0);    
  transform-origin: top;
  transition: transform 0.26s ease;
}
p:hover ~ ul {
  transform: scaleY(1);
}

<p>Hover This</p>
<ul>
  <li>Coffee</li>
  <li>Tea</li>
  <li>Milk</li>
</ul>

ul {
  clip: rect(auto, auto, 0, auto);
  position: absolute;
  margin: -1rem 0;
  padding: .5rem;

  color: white;

  background-color: rgba(0, 0, 0, 0.8);

  transition-property: clip;
  transition-duration: 0.5s;
  transition-timing-function: cubic-bezier(0.175, 0.885, 0.32, 1.275);
}
h3:hover ~ ul,
h3:active ~ ul,
ul:hover {
  clip: rect(auto, auto, 10rem, auto);
}

<h3>Hover here</h3>
<ul>
  <li>This list</li>
  <li>is clipped.</li>
  <li>A clip transition</li>
  <li>will show it</li>
</ul>
<p>
  Some text...
</p>

window.toggleExpand = function(element) {
    if (!element.style.height || element.style.height == '0px') { 
        element.style.height = Array.prototype.reduce.call(element.childNodes, function(p, c) {return p + (c.offsetHeight || 0);}, 0) + 'px';
    } else {
        element.style.height = '0px';
    }
}

#menu #list {
    height: 0px;
    transition: height 0.3s ease;
    background: #d5d5d5;
    overflow: hidden;
}

<div id="menu">
    <input value="Toggle list" type="button" onclick="toggleExpand(document.getElementById('list'));">
    <ul id="list">
        <!-- Works well with dynamically-sized content. -->
        <li>item</li>
        <li><div style="height: 100px; width: 100px; background: red;"></div></li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
    </ul>
</div>

/* final display */
#menu #list {
    margin: .5em 1em;
    padding: 1em;
}

/* hide */
#menu:not(:hover) #list {
    font-size: 0;
    margin: 0;
    opacity: 0;
    padding: 0;
    /* fade out, then shrink */
    transition: opacity .25s,
                font-size .5s .25s,
                margin .5s .25s,
                padding .5s .25s;
}

/* reveal */
#menu:hover #list {
    /* unshrink, then fade in */
    transition: font-size .25s,
                margin .25s,
                padding .25s,
                opacity .5s .25s;
}

<div id="menu">
    <b>hover me</b>
    <ul id="list">
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
    </ul>
</div>

<p>Another paragraph...</p>

.outer {
  border: dashed red 1px;
  position: relative;
}

.dummy {
  visibility: hidden;
}

.real {
  position: absolute;
  background: yellow;
  height: 0;
  transition: height 0.5s;
  overflow: hidden;
}

.outer:hover>.real {
  height: 100%;
}

Hover over the box below:
<div class="outer">
  <!-- The actual element that you'd like to animate -->
  <div class="real">
unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable
content unpredictable content unpredictable content unpredictable content
  </div>
  <!-- An exact copy of the element you'd like to animate. -->
  <div class="dummy" aria-hidden="true">
unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable content unpredictable
content unpredictable content unpredictable content unpredictable content
  </div>
</div>

body {
  font-family: sans-serif;
}

.toggle {
  position: relative;
  border: 2px solid #333;
  border-radius: 3px;
  margin: 5px;
  width: 200px;
}

.toggle-header {
  margin: 0;
  padding: 10px;
  background-color: #333;
  color: white;
  text-align: center;
  cursor: pointer;
}

.toggle-height {
  background-color: tomato;
  overflow: hidden;
  transition: max-height .6s ease;
  max-height: 0;
}

.toggle:hover .toggle-height {
  max-height: 1000px;
}

.toggle-transform {
  padding: 5px;
  color: white;
  transition: transform .4s ease;
  transform: translateY(-100%);
}

.toggle:hover .toggle-transform {
  transform: translateY(0);
}

<div class="toggle">
  <div class="toggle-header">
    Toggle!
  </div>
  <div class="toggle-height">
    <div class="toggle-transform">
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
    </div>
  </div>
</div>

<div class="toggle">
  <div class="toggle-header">
    Toggle!
  </div>
  <div class="toggle-height">
    <div class="toggle-transform">
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
    </div>
  </div>
</div>

document.querySelector("button").addEventListener(
  "click", 
  function(){
    document.querySelector("div").classList.toggle("hide");
  }
)

div {    
    max-height: 20000px;
    transition: max-height 3000ms;
    overflow-y: hidden;
}

.hide {
    max-height: 0;
    transition: none;
}

<button>Toggle</button>
<div class="hide">Lorem ipsum dolor sit amet, ius solet dignissim honestatis ad. Mea quem tibique intellegat te. Insolens deterruisset cum ea. Te omnes percipit consulatu eos. Vix novum primis salutatus no, eam denique sensibus et, his ipsum senserit ne. Lorem ipsum dolor sit amet, ius solet dignissim honestatis ad. Mea quem tibique intellegat te. Insolens deterruisset cum ea. Te omnes percipit consulatu eos. Vix novum primis salutatus no, eam denique sensibus et, his ipsum senserit ne. Lorem ipsum dolor sit amet, ius solet dignissim honestatis ad. Mea quem tibique intellegat te. Insolens deterruisset cum ea. Te omnes percipit consulatu eos. Vix novum primis salutatus no, eam denique sensibus et, his ipsum senserit ne. Lorem ipsum dolor sit amet, ius solet dignissim honestatis ad. Mea quem tibique intellegat te. Insolens deterruisset cum ea. Te omnes percipit consulatu eos. Vix novum primis salutatus no, eam denique sensibus et, his ipsum senserit ne. Lorem ipsum dolor sit amet, ius solet dignissim honestatis ad. Mea quem tibique intellegat te. Insolens deterruisset cum ea. Te omnes percipit consulatu eos. Vix novum primis salutatus no, eam denique sensibus et, his ipsum senserit ne. Lorem ipsum dolor sit amet, ius solet dignissim honestatis ad. Mea quem tibique intellegat te. Insolens deterruisset cum ea. Te omnes percipit consulatu eos. Vix novum primis salutatus no, eam denique sensibus et, his ipsum senserit ne. Lorem ipsum dolor sit amet, ius solet dignissim honestatis ad. Mea quem tibique intellegat te. Insolens deterruisset cum ea. Te omnes percipit consulatu eos. Vix novum primis salutatus no, eam denique sensibus et, his ipsum senserit ne. Lorem ipsum dolor sit amet, ius solet dignissim honestatis ad. Mea quem tibique intellegat te. Insolens deterruisset cum ea. Te omnes percipit consulatu eos. Vix novum primis salutatus no, eam denique sensibus et, his ipsum senserit ne. Lorem ipsum dolor sit amet, ius solet dignissim honestatis ad. Mea quem tibique intellegat te. Insolens deterruisset cum ea. Te omnes percipit consulatu eos. Vix novum primis salutatus no, eam denique sensibus et, his ipsum senserit ne. 
</div>

$(function () {
  $(".toggler").click(function () {
    $(this).next().toggleClass("collapsed");
    $(this).toggleClass("toggled"); // this just rotates the expander arrow
  });
});

.collapsible-wrapper {
  display: flex;
  overflow: hidden;
}
.collapsible-wrapper:after {
  content: '';
  height: 50px;
  transition: height 0.3s linear, max-height 0s 0.3s linear;
  max-height: 0px;
}
.collapsible {
  transition: margin-bottom 0.3s cubic-bezier(0, 0, 0, 1);
  margin-bottom: 0;
  max-height: 1000000px;
}
.collapsible-wrapper.collapsed > .collapsible {
  margin-bottom: -2000px;
  transition: margin-bottom 0.3s cubic-bezier(1, 0, 1, 1),
              visibility 0s 0.3s, max-height 0s 0.3s;
  visibility: hidden;
  max-height: 0;
}
.collapsible-wrapper.collapsed:after
{
  height: 0;
  transition: height 0.3s linear;
  max-height: 50px;
}

/* END of the collapsible implementation; the stuff below
   is just styling for this demo */

#container {
  display: flex;
  align-items: flex-start;
  max-width: 1000px;
  margin: 0 auto;
}  


.menu {
  border: 1px solid #ccc;
  box-shadow: 0 1px 3px rgba(0,0,0,0.5);
  margin: 20px;

  
}

.menu-item {
  display: block;
  background: linear-gradient(to bottom, #fff 0%,#eee 100%);
  margin: 0;
  padding: 1em;
  line-height: 1.3;
}
.collapsible .menu-item {
  border-left: 2px solid #888;
  border-right: 2px solid #888;
  background: linear-gradient(to bottom, #eee 0%,#ddd 100%);
}
.menu-item.toggler {
  background: linear-gradient(to bottom, #aaa 0%,#888 100%);
  color: white;
  cursor: pointer;
}
.menu-item.toggler:before {
  content: '';
  display: block;
  border-left: 8px solid white;
  border-top: 8px solid transparent;
  border-bottom: 8px solid transparent;
  width: 0;
  height: 0;
  float: right;
  transition: transform 0.3s ease-out;
}
.menu-item.toggler.toggled:before {
  transform: rotate(90deg);
}

body { font-family: sans-serif; font-size: 14px; }

*, *:after {
  box-sizing: border-box;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<div id="container">
  <div class="menu">
    <div class="menu-item">Something involving a holodeck</div>
    <div class="menu-item">Send an away team</div>
    <div class="menu-item toggler">Advanced solutions</div>
    <div class="collapsible-wrapper collapsed">
      <div class="collapsible">
        <div class="menu-item">Separate saucer</div>
        <div class="menu-item">Send an away team that includes the captain (despite Riker's protest)</div>
        <div class="menu-item">Ask Worf</div>
        <div class="menu-item">Something involving Wesley, the 19th century, and a holodeck</div>
        <div class="menu-item">Ask Q for help</div>
      </div>
    </div>
    <div class="menu-item">Sweet-talk the alien aggressor</div>
    <div class="menu-item">Re-route power from auxiliary systems</div>
  </div>

  <div class="menu">
    <div class="menu-item">Something involving a holodeck</div>
    <div class="menu-item">Send an away team</div>
    <div class="menu-item toggler">Advanced solutions</div>
    <div class="collapsible-wrapper collapsed">
      <div class="collapsible">
        <div class="menu-item">Separate saucer</div>
        <div class="menu-item">Send an away team that includes the captain (despite Riker's protest)</div>
      </div>
    </div>
    <div class="menu-item">Sweet-talk the alien aggressor</div>
    <div class="menu-item">Re-route power from auxiliary systems</div>
  </div>

  <div class="menu">
    <div class="menu-item">Something involving a holodeck</div>
    <div class="menu-item">Send an away team</div>
    <div class="menu-item toggler">Advanced solutions</div>
    <div class="collapsible-wrapper collapsed">
      <div class="collapsible">
        <div class="menu-item">Separate saucer</div>
        <div class="menu-item">Send an away team that includes the captain (despite Riker's protest)</div>
        <div class="menu-item">Ask Worf</div>
        <div class="menu-item">Something involving Wesley, the 19th century, and a holodeck</div>
        <div class="menu-item">Ask Q for help</div>
        <div class="menu-item">Separate saucer</div>
        <div class="menu-item">Send an away team that includes the captain (despite Riker's protest)</div>
        <div class="menu-item">Ask Worf</div>
        <div class="menu-item">Something involving Wesley, the 19th century, and a holodeck</div>
        <div class="menu-item">Ask Q for help</div>
      </div>
    </div>
    <div class="menu-item">Sweet-talk the alien aggressor</div>
    <div class="menu-item">Re-route power from auxiliary systems</div>
  </div>

</div>

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  transition: 0.25s;
  font-family: monospace;
}

body {
  margin: 10px 0 0 10px;
}

.box {
  width: 150px;
  height: 150px;
  margin: 0 2px 10px 0;
  background: #2d333b;
  border: solid 10px #20262e;
  overflow: hidden;
  display: inline-flex;
  flex-direction: column;
}

.space {
  flex-basis: 100%;
  flex-grow: 1;
  flex-shrink: 0;    
}

p {
  flex-basis: auto;
  flex-grow: 0;
  flex-shrink: 1;
  background: #20262e;
  padding: 10px;
  width: 100%;
  text-align: left;
  color: white;
}

.box:hover .space {
  flex-grow: 0;
  flex-shrink: 1;
}
  
.box:hover p {
  flex-grow: 1;
  flex-shrink: 0;    
}

<div class="box">
  <div class="space"></div>
  <p>
    Super Metroid Prime Fusion
  </p>
</div>
<div class="box">
  <div class="space"></div>
  <p>
    Resident Evil 2 Remake
  </p>
</div>
<div class="box">
  <div class="space"></div>
  <p>
    Yolo The Game
  </p>
</div>
<div class="box">
  <div class="space"></div>
  <p>
    Final Fantasy 7 Remake + All Additional DLC + Golden Tophat
  </p>
</div>
<div class="box">
  <div class="space"></div>
  <p>
    DerpVille
  </p>
</div>

const showHideElement = (element, open) => {
  element.style.height = element.scrollHeight + 'px';
  element.classList.toggle('open', open);

  if (open) {
    element.addEventListener('transitionend', () => {
      element.style.removeProperty('height');
    }, {
      once: true
    });
  } else {
    window.setTimeout(() => {
      element.style.removeProperty('height');
    });
  }
}

const menu = document.body.querySelector('#menu');
const list = document.body.querySelector('#menu > ul')

menu.addEventListener('mouseenter', () => showHideElement(list, true));
menu.addEventListener('mouseleave', () => showHideElement(list, false));

#menu > ul {
  height: 0;
  overflow: hidden;
  background-color: #999;
  transition: height .25s ease-out;
}

#menu > ul.open {
  height: auto;
}

<div id="menu">
  <a>hover me</a>
  <ul>
    <li>item</li>
    <li>item</li>
    <li>item</li>
    <li>item</li>
    <li>item</li>
  </ul>
</div>

body {
  padding: 1em;
}

.trigger {
  font-weight: bold;
}

/* .expander is there for float clearing purposes only */
.expander::after {
  content: '';
  display: table;
  clear: both;
}

.outer {
  float: left; /* purpose: shrink to fit content */
  border: 1px solid green;
  overflow: hidden;
}

.inner {
  transition: padding-bottom 0.3s ease-in-out;  /* or whatever crazy transition function you can come up with! */
  padding-bottom: 0%;  /* percentage padding is defined in terms of width. The width at this level is equal to the height of the content */
  height: 0;

  /* unfortunately, change of writing mode has other bad effects like orientation of cursor */
  writing-mode: vertical-rl;
  cursor: default; /* don't want the vertical-text (sideways I-beam) */
  transform: rotate(-90deg) translateX(-100%);  /* undo writing mode */
  transform-origin: 0 0;
  margin: 0;  /* left/right margins here will add to height */
}

.inner > div { white-space: nowrap; }

.expander:hover .inner,  /* to keep open when expanded */
.trigger:hover+.expander .inner {
  padding-bottom: 100%;
}

<div class="trigger">HoverMe</div>
<div class="expander">
  <div class="outer">
    <div class="inner">
      <div>First Item</div>
      <div>Content</div>
      <div>Content</div>
      <div>Content</div>
      <div>Long Content can't be wider than outer height unfortunately</div>
      <div>Last Item</div>
    </div>
  </div>
</div>
<div>
  after content</div>
</div>

#child0 {
    display: none;
}
#parent0:hover #child0 {
    display: block;
    animation: height-animation;
    animation-duration: 200ms;
    animation-timing-function: linear;
    animation-fill-mode: backwards;
    animation-iteration-count: 1;
    animation-delay: 200ms;
}
@keyframes height-animation {
    0% {
        clip-path: polygon(0% 0%, 100% 0.00%, 100% 0%, 0% 0%);
    }
    100% {
        clip-path: polygon(0% 0%, 100% 0.00%, 100% 100%, 0% 100%);
    }
}

<div id="parent0">
    <h1>Hover me (height: 0)</h1>
    <div id="child0">Some content
        <br>Some content
        <br>Some content
        <br>Some content
        <br>Some content
        <br>Some content
        <br>
    </div>
</div>

.parent{ border-top: #999 1px solid;}
h1{ margin: .5rem; font-size: 1.3rem}
.children {
  height: 0;
  overflow: hidden;
  background-color: #dedede;
  transition: padding .2s ease-in-out, opacity .2s ease-in-out;
  padding: 0 .5rem;
  opacity: 0;
}
.children::before, .children::after{ content: "";display: block;}
.children::before{ margin-top: -2rem;}
.children::after{ margin-bottom: -2rem;}
.parent:hover .children {
  height: auto;
  opacity: 1;
  padding: 2.5rem .5rem;/* 0.5 + abs(-2), make sure it's less than expected min-height */
}

<div class="parent">
  <h1>Hover me</h1>
  <div class="children">Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>
  </div>
</div>
<div class="parent">
  <h1>Hover me(long content)</h1>
  <div class="children">Some content
    <br>Some content<br>Some content
    <br>Some content<br>Some content
    <br>Some content<br>Some content
    <br>Some content<br>Some content
    <br>Some content<br>Some content
    <br>
  </div>
</div>
<div class="parent">
  <h1>Hover me(short content)</h1>
  <div class="children">Some content
    <br>Some content
    <br>Some content
    <br>
  </div>
</div>

[...document.querySelectorAll('.ab')].forEach(wrapper => {
    wrapper.addEventListener('click', function () {
        this.classList.toggle('active');
    });
});

* {
  margin: 0;
  box-sizing: border-box;
}

.c {
  overflow: hidden;
}

.items {
  width: 100%;
  visibility: hidden;
  max-height: 0;
  margin-bottom: -2000px;
  -webkit-transition: margin 0.6s cubic-bezier(1, 0, 1, 1), max-height 0s 0.6s linear, visibility 0s 0.6s linear;
  transition: margin 0.6s cubic-bezier(1, 0, 1, 1), max-height 0s 0.6s linear, visibility 0s 0.6s linear;
}
.items > * {
  padding: 1rem;
  background-color: #ddd;
  -webkit-transition: background-color 0.6s ease;
  transition: background-color 0.6s ease;
}
.items > *:hover {
  background-color: #eee;
}

.ab {
  padding: 1rem;
  cursor: pointer;
  background: #eee;
}
.ab.active + .c .items {
  max-height: 2000px;
  margin-bottom: 0;
  visibility: visible;
  -webkit-transition: max-height 0.6s cubic-bezier(1, 0, 1, 1);
  transition: max-height 0.6s cubic-bezier(1, 0, 1, 1);
}

.dropdown {
  margin-right: 1rem;
}

.wrapper {
  display: -webkit-box;
  display: flex;
}

<div class="wrapper">
    <div class="dropdown">
        <div class="ab">just text</div>
        <div class="ab">just text</div>
        <div class="ab">dropdown</div>
        <div class="c">
            <div class="items">
                <p>items</p>
                <p>items</p>
                <p>items asl;dk l;kasl;d sa;lk</p>
                <p>items sal;kd</p>
                <p>items</p>
            </div>
        </div>
        <div class="ab">just text</div>
        <div class="ab">just text</div>
    </div>
    
    <div class="dropdown">
        <div class="ab">dropdown</div>
        <div class="c">
            <div class="items">
                <p>items</p>
                <p>items</p>
                <p>items</p>
                <p>items</p>
                <p>items</p>
                <p>items</p>
                <p>items</p>
                <p>items</p>
                <p>items</p>
                <p>items</p>
                <p>items</p>
            </div>
        </div>
        <div class="ab">text</div>
    </div>
    
    <div class="dropdown">
        <div class="ab">placeholder</div>
        <div class="ab">dropdown</div>
        <div class="c">
            <div class="items">
                <p>items</p>
                <p>items</p>
            </div>
        </div>
        <div class="ab">placeholder</div>
        <div class="ab">placeholder</div>
        <div class="ab">placeholder</div>
    </div>
</div>
<h1>text to be pushed</h1>

body {
  background-color: linen;
}

main {
  background-color: white;
}

[id^="toggle_"] ~ .content {
  line-height: 0;
  opacity: 0;
  padding: 0 .5rem;
  transition: .2s ease-out;
}

[id^="toggle_"] ~ .content > p {
  margin: 0;
    transition: .2s ease-out;
}

[id^="toggle_"]:checked ~ .content   {
  opacity: 1;
  padding: .5rem;
  line-height: 1.5;
}

[id^="toggle_"]:checked ~ .content p  {
    margin-bottom: .75rem;
}

[id^="toggle_"] + label {
  display: flex;
  justify-content: space-between;
  padding: 0.5em 1em;
  background: lightsteelblue;
  border-bottom: 1px solid gray;
  cursor: pointer;
}

[id^="toggle_"] + label:before {
  content: "Show";
}

[id^="toggle_"]:checked + label:before {
  content: "Hide";
}

[id^="toggle_"] + label:after {
  content: "\25BC";
}

[id^="toggle_"]:checked + label:after {
  content: "\25B2";
}

<main>
    <div>
        <input type="checkbox" id="toggle_1" hidden>
        <label for="toggle_1" hidden></label>
        <div class="content">
            <p>
                Lorem ipsum dolor sit amet, consectetur adipiscing elit. Duis dolor neque, commodo quis leo ut, auctor tincidunt mauris. Nunc fringilla tincidunt metus, non gravida lorem condimentum non. Duis ornare purus nisl, at porta arcu eleifend eget. Integer lorem ante, porta vulputate dui ut, blandit tempor tellus. Proin facilisis bibendum diam, sit amet rutrum est feugiat ut. Mauris rhoncus convallis arcu in condimentum. Donec volutpat dui eu mollis vulputate. Nunc commodo lobortis nunc at ultrices. Suspendisse in lobortis diam. Suspendisse eget vestibulum ex.
            </p>
        </div>
    </div>
    <div>
        <input type="checkbox" id="toggle_2" hidden>
        <label for="toggle_2" hidden></label>
        <div class="content">
            <p>
                Maecenas laoreet nunc sit amet nulla ultrices auctor. Vivamus sed nisi vitae nibh condimentum pulvinar eu vel lorem. Sed pretium viverra eros ut facilisis. In ut fringilla magna. Sed a tempor libero. Donec sapien libero, lacinia sed aliquet ut, imperdiet finibus tellus. Nunc tellus lectus, rhoncus in posuere quis, tempus sit amet enim. Morbi et erat ac velit fringilla dignissim. Donec commodo, est id accumsan cursus, diam dui hendrerit nisi, vel hendrerit purus dolor ut risus. Phasellus mattis egestas ipsum sed ullamcorper. In diam ligula, rhoncus vel enim et, imperdiet porta justo. Curabitur vulputate hendrerit nisl, et ultricies diam. Maecenas ac leo a diam cursus ornare nec eu quam.
            </p>
            <p>Sed non vulputate purus, sed consectetur odio. Sed non nibh fringilla, imperdiet odio nec, efficitur ex. Suspendisse ut dignissim enim. Maecenas felis augue, tempor sit amet sem fringilla, accumsan fringilla nibh. Quisque posuere lacus tortor, quis malesuada magna elementum a. Nullam id purus in ante molestie tincidunt. Morbi luctus orci eu egestas dignissim. Sed tincidunt, libero quis scelerisque bibendum, ligula nisi gravida libero, id lacinia nulla leo in elit.
            </p>
            <p>Aenean aliquam risus id consectetur sagittis. Aliquam aliquam nisl eu augue accumsan, vel maximus lorem viverra. Aliquam ipsum dolor, tempor et justo ac, fermentum mattis dui. Etiam at posuere ligula. Vestibulum tortor metus, viverra vitae mi non, laoreet iaculis purus. Praesent vel semper nibh. Curabitur a congue lacus. In et pellentesque lorem. Morbi posuere felis non diam vulputate, non vulputate ex vehicula. Vivamus ultricies, massa id sagittis consequat, sem mauris tincidunt nunc, eu vehicula augue quam ut mauris.
            </p>
        </div>
    </div>
        <div>
        <input type="checkbox" id="toggle_3" hidden>
        <label for="toggle_3" hidden></label>
        <div class="content">
            <p>
                Lorem ipsum dolor sit amet, consectetur adipiscing elit. Duis dolor neque, commodo quis leo ut, auctor tincidunt mauris. Nunc fringilla tincidunt metus, non gravida lorem condimentum non. Duis ornare purus nisl, at porta arcu eleifend eget. Integer lorem ante, porta vulputate dui ut, blandit tempor tellus. Proin facilisis bibendum diam, sit amet rutrum est feugiat ut. Mauris rhoncus convallis arcu in condimentum. Donec volutpat dui eu mollis vulputate. Nunc commodo lobortis nunc at ultrices. Suspendisse in lobortis diam. Suspendisse eget vestibulum ex.
            </p>
            <p>Sed non vulputate purus, sed consectetur odio. Sed non nibh fringilla, imperdiet odio nec, efficitur ex. Suspendisse ut dignissim enim. Maecenas felis augue, tempor sit amet sem fringilla, accumsan fringilla nibh. Quisque posuere lacus tortor, quis malesuada magna elementum a. Nullam id purus in ante molestie tincidunt. Morbi luctus orci eu egestas dignissim. Sed tincidunt, libero quis scelerisque bibendum, ligula nisi gravida libero, id lacinia nulla leo in elit.
            </p>
        </div>
    </div>
</main>

#page {
  font-size: calc( (35vw + 65vh) / 30); /* just some responsive design as a bonus */
}
#slidebox {
  background-color: #e8e8e8;
  visibility: hidden;
  font-size: 0; /* animated from 0 to 100% */
  opacity: 0; /* optional */
  transition: 0.5s;
}
a#ahover:hover ~ #slidebox {
  visibility: visible;
  font-size: 100%; /* animated from 0 to 100% */
  opacity: 1; /* optional */
}
a#ahover:not(:hover) ~ #slidebox * {
  border-top: 0;
  border-bottom: 0;
  /* Put here anything vertical that uses px as unit, in this case the borders */
}
a#button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  position: relative;
  width: 20em;
  height: 3em;
  padding: 0.5em;
  border: 2px solid #0080ff;
  border-radius: 0.4em;
  background-color: #8fbfef;
  color: #404040;
  box-sizing: border-box;
}
#someform {
  margin-top: 1em;
  margin-bottom: 1em;
  padding: 1em 4em 1em 4em;
  background-color: #d8ffd8;
  border: 1px solid #888888;
}
#someform input {
  display: inline-block;
  box-sizing: border-box;
  font-size: 125%;
  padding: 0.5em;
  width: 50%; /* anything horizontal can still be in px or % */
  border-radius: 0.4em;
  border: 1px solid red;
}

<div id=page>
  <a id=ahover href="#">Hover me</a><br>
  Here is the box that slides:
  <div id=slidebox>
    I am the content of the slide box (line1).<br>
    I am the content of the slide box (line2).<br>
    <a id=button href="#">I am some button in the slide box</a><br>
    I am the content of the slide box (line3).<br>
    I am the content of the slide box (line4).
    <div id=someform>
      Some box with a form or anything...<br>
      <input type=text value="Text Box">
    </div>
    I am the content of the slide box (line5).<br>
    I am the content of the slide box (line6).
  </div>
  And this is after the box.
</div>

const switch_ele = document.getElementById('aclass');
switch_ele.addEventListener('click', function(){
    const box_ele = document.getElementById('slidebox');
    box_ele.className = box_ele.className == 'show' ? 'hide' : 'show';
}, true);

#page {
  font-size: calc( (35vw + 65vh) / 30); /* just some responsive design as a bonus */
}
#slidebox {
  background-color: #e8e8e8;
  visibility: hidden;
  font-size: 0; /* animated from 0 to 100% */
  opacity: 0; /* optional */
  transition: .5s;
}
#slidebox.show {
  visibility: visible;
  font-size: 100%; /* animated from 0 to 100% */
  opacity: 1; /* optional */
}
#slidebox.hide * {
  border-top: 0;
  border-bottom: 0;
  /* Put here anything vertical that uses px as unit, in this case the borders */
}
a#button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  position: relative;
  width: 20em;
  height: 3em;
  padding: 0.5em;
  border: 2px solid #0080ff;
  border-radius: 0.4em;
  background-color: #8fbfef;
  color: #404040;
  box-sizing: border-box;
}
#someform {
  margin-top: 1em;
  margin-bottom: 1em;
  padding: 1em 4em 1em 4em;
  background-color: #d8ffd8;
  border: 1px solid #888888;
}
#someform input {
  display: inline-block;
  box-sizing: border-box;
  font-size: 125%;
  padding: 0.5em;
  width: 50%; /* anything horizontal can still be in px or % */
  border-radius: 0.4em;
  border: 1px solid red;
}

<div id=page>
  <a id=aclass href="#">Switch class w/ js</a><br>
  Here is the box that slides:
  <div id=slidebox class=hide>
    I am the content of the slide box (line1).<br>
    I am the content of the slide box (line2).<br>
    <a id=button href="#">I am some button in the slide box</a><br>
    I am the content of the slide box (line3).<br>
    I am the content of the slide box (line4).
    <div id=someform>
      Some box with a form or anything...<br>
      <input type=text value="Text Box">
    </div>
    I am the content of the slide box (line5).<br>
    I am the content of the slide box (line6).
  </div>
  And this is after the box.
</div>

.demo01 {
  overflow: hidden;
  position: absolute;
  pointer-events: none;
}
.demo01__content {
  background: lightgray;
  padding: 1rem;
  pointer-events: all;
  transform: translateY(-100%);
  transition: transform 1s, visibility 1s;
  visibility: hidden;
}
:checked ~ .demo01 .demo01__content {
  transform: translateY(0);
  visibility: visible;
}

<input type="checkbox" /> ⬅︎ Toggle
<div>Something before 🙃</div>
<div class="demo01">
  <div class="demo01__content">
    This content should…<br />
    toggle! 👻
  </div>
</div>
<div>Something after 🙂</div>

.demo02 {
  display: grid;
  grid-template-rows: 0fr;
  overflow: hidden;
  transition: grid-template-rows 1s;
}
.demo02__content {
  align-self: end;
  min-height: 0;
  background: lightgray;
  transition: visibility 1s;
  visibility: hidden;
}
.demo02__padding {
  padding: 1rem;
}
:checked ~ .demo02 {
  grid-template-rows: 1fr;
}
:checked ~ .demo02 .demo02__content {
  visibility: visible;
}

<input type="checkbox" /> ⬅︎ Toggle
<div>Something before 🙃</div>
<div class="demo02">
  <div class="demo02__content">
    <div class="demo02__padding">
      This content should…<br />
      toggle! 👻
    </div>
  </div>
</div>
<div>Something after 🙂</div>

.grid-container {
  display: grid;
  position: absolute;
}

.content {
  background: aqua;
  height: 0;
  overflow: hidden;
  transition: 1s;
}

span:hover + .grid-container .content {
  height: 100%;
}

<span>Hover over me!</span>

<div class="grid-container">

  <div class="content">Lorem ipsum dolor sit amet, consectetur adipiscing 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. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</div>

</div>

<p>Rest of the page content...</p>

.grid-container {
  display: grid;
  position: absolute;
  overflow: hidden;
  pointer-events: none; /* to enable interaction with elements below the container */
}

.content {
  background: aqua;
  pointer-events: auto;
  position: relative;
  top: -100%;
  transition: 1s;
}

span:hover + .grid-container .content {
  top: 0;
}

<span>Hover over me!</span>

<div class="grid-container">

  <div class="content">Lorem ipsum dolor sit amet, consectetur adipiscing 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. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</div>

</div>

<p>Rest of the page content...</p>

.outer-container { height: 0; display: flex; flex-direction: column; }

.inner-container { display: flex; flex-direction: column; }

.hidden-content { 
    height: 0; 
    opacity: 0; 
    transition: height 1s 0.5s ease-in-out, opacity 0.5s ease-in-out;
    /* transition out: first fade out opacity, then shrink height after a delay equal to the opacity duration */
    }

.trigger:hover + .inner-container > .hidden-content { 
    height: 100%; 
    opacity: 1; 
    transition: height 1s ease-in-out, opacity 0.5s 1s ease-in-out;
    /* transition in: first expand height, then fade in opacity after a delay equal to the height duration */
}

<div class="outer-container">
  <a href="#" class="trigger">Hover to Reveal Inner Container's Hidden Content</a>
  <div class="inner-container">
    <div class="hidden-content">This is hidden content. When triggered by hover, its height transitions from 0 to 100%, which pushes other content in the same container down gradually.</div>
    <div>Within the same container, this other content is pushed down gradually as the hidden content's height transitions from 0 to 100%.</div>
  </div>
</div>

function collapse(el, collapse) {
    el.style.height = el.scrollHeight+'px';
    if(!collapse)
        requestAnimationFrame(() => el.style.height = '0px');
}
     
function toggle(btn) {
    const ul = btn.nextElementSibling,
      collapsed = ul.classList.contains('collapsed');
      
    collapse(ul, collapsed);
    
    ul.classList.toggle('collapsed');
}

ul {
    transition: height 0.16s ease-out;
    overflow:hidden;
    height:auto;
}

<ul>
  <li>Item 1
  <li>Item 2
  <li>Item 3
</ul>
<button onclick="toggle(this)">Toggle</button>
<ul>
  <li>Item 4
  <li>Item 5
  <li>Item 6
</ul>
<ul>
  <li>Item 7
  <li>Item 8
  <li>Item 9
</ul>

#parent0 {
  display: grid;
  grid-template-rows: min-content 0fr;
  transition: grid-template-rows 500ms;
}
#child0 {   
  background-color: #dedede;
  overflow: hidden;
}
#parent0:hover{
  grid-template-rows: min-content 1fr;
}

<div id="parent0">
  <h1>Hover me</h1>
  <div id="child0">Some content
    <br>Some content
    <br>Some content
    <br>Some content
    <br>Some content
  </div>
</div>

$(document).ready(function() {
  $('.nav_menu .menu-item-has-children').each(function(index) {
    var smheight = Math.ceil($(this).find(".sub-menu").outerHeight());
    $(this).find(".sub-menu").height(0);
    $(this).mouseenter(function() {
      $(this).find(".sub-menu").animate({
        height: smheight + "px"
      }, 250);
    });
  });
  $('.nav_menu .menu-item-has-children').each(function(index) {
    $(this).mouseleave(function() {
      $(this).find(".sub-menu").animate({
        height: "0"
      }, 250);
    });
  });
});

.menu-item-has-children {
  background-color: violet;
}
li ul {
  overflow: hidden;
  background-color: lime;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<ul class="nav_menu">
  <li class="menu-item-has-children">
    <a href="#">Main Thing</a>
    <ul class="sub-menu">
      <li><a href="#">Sub Thing</a></li>
      <li><a href="#">Sub Thing</a></li>
      <li><a href="#">Sub Thing</a></li>
    </ul>
  </li>
  <li class="menu-item-has-children">
    <a href="#">Main Thing</a>
    <ul class="sub-menu">
      <li><a href="#">Sub Thing</a></li>
      <li><a href="#">Sub Thing</a></li>
      <li><a href="#">Sub Thing</a></li>
    </ul>
  </li>
</ul>