How to have smooth movements when inserting a new element?

Question

I insert a new div element in the middle of the others, and I want the others to move to their new position in a fluid way and not suddenly as is currently the case.

Here is my code in action: https://codepen.io/BigEiffelTower/pen/vYBgqZq

<div id="top">
  Top of the page
</div>

<div id="middle">

</div>

<div id="bottom">
  <form> 
    <input type="button" value="Add an element">
  </form>
  End of the page
</div>

#top, #middle, #bottom {
  font-size: 1.5em;
  border: 2px solid black;
  margin: 0.3em
  transition: all 2s ease;
}

var btn = document.querySelector('input');
btn.addEventListener('click', updateBtn);

function updateBtn() {
    $el = $("<div>", {
            id: 'middle-item',
            text: "New element"
        });
    $("#middle").append($el);
}

How to make the #three element move smoothly when you click on the button?

Answer 1

You can do it with animate() function.

var btn = document.querySelector("input");
btn.addEventListener("click", updateBtn);

function updateBtn() {
  $el = $("<div>", {
    id: "middle-item",
    text: "New element",
    style: "display: none"
  });
  $("#middle")
    .append($el)
    .animate({ height: "+=" + $el.height() }, 200, function() {
      $el.fadeIn(100);
    });
}

/* You don't need to define a `css transition` anymore. */
#top, #middle, #bottom {
  font-size: 1.5em;
  border: 2px solid black;
  margin: 0.3em
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<div id="top">
  Top of the page
</div>

<div id="middle">

</div>

<div id="bottom">
  <form>
    <input type="button" value="Add an element">
  </form>
  End of the page
</div>