Display:inline-block before transition with pure Javascript

I am trying to animate a hidden menu box to appear and slide down when the mouse is over the button. It works properly, but when adding the transition display:inline-block -> display:none -> display:inline-block the transition disappears.

I already saw a few posts with solutions in jQuery, but none is pure JavaScript, if not for one using EventListener (that I was not able to apply to my case).

I tried to separate the JS in two functions, one to show the block, and and one to change the class. It did not work. One way would be to add something like an EventListener that would call a function after the display property is changed (just adding it in JS seems not sufficient). Any help is appreciated.

Here the code:

function overbutton(divname) {
  element = document.getElementById(divname);
  if (element.className == "menu_hidden menu_about") {
    //element.style.display ="inline-block";
    element.className = "menu_shown menu_about";
  } else {
    //element.style.display ="none";
    element.className = "menu_hidden menu_about";
  }
}

function onmenu(divname) {
  //element.style.display ="inline-block";
  element = document.getElementById(divname);
  element.className = "menu_shown menu_about";
}

function outmenu(divname) {
  //element.style.display ="none";
  element = document.getElementById(divname);
  element.className = "menu_hidden menu_about";
}

.menu_about {
  background-color: white;
  height: 100px;
  width: 100px;
  top: 20px;
  left: 50px;
  border-radius: 10px;
  box-shadow: 0px 0px 10px #2B2B2B40;
  -webkit-box-shadow: 0px 0px 10px #2B2B2B40;
  -moz-box-shadow: 0px 0px 10px #2B2B2B40;
  position: absolute;
  padding: 20px;
}

.menu_hidden {
  -webkit-transform: translate(0%, -5px);
  transform: translate(0%, -5px);
  transition: transform 0.3s;
  -webkit-transition: -webkit-transform 0.3s;
}

.menu_shown {
  -webkit-transform: translate(0%, 5px);
  transform: translate(0%, 10px);
  transition: transform 0.3s;
  -webkit-transition: -webkit-transform 0.3s;
}

<div style="padding-left:100px">

  <button class="button-menu" onmouseover="overbutton('about_menu_div')" onmouseout="overbutton('about_menu_div')">
    menu Button
  </button>

  <!-- add style="display:none;"-->
  <div id="about_menu_div" class="menu_hidden menu_about" onmouseover="onmenu('about_menu_div')" onmouseout="outmenu('about_menu_div')">
    Menu
  </div>

</div>

JSfiddle: https://jsfiddle.net/g0ktx574/

Answer

No need for any JavaScript in this case. You can solve this by using CSS, and more specifically, the :hover pseudo-class and the + adjacent sibling selector.

Also, the display property cannot be animated. Instead use opacity in combination with visibility.

.wrapper {
  position: relative;
}

.menu_about {
  position: absolute;
  top: 100%;
  left: 0;
  background-color: white;
  height: 100px;
  width: 100px;
  padding: 20px;
  border-radius: 10px;
  box-shadow: 0px 0px 10px #2B2B2B40;
  opacity: 0;
  visibility: hidden;
  transition-property: opacity visibility transform;
  transition: 0.3s;
}

.button-menu:hover + .menu_about,
.menu_about:hover {
  opacity: 1;
  visibility: visible;
  transform: translate(0%, 5px);
}

<div class="wrapper">
  <button class="button-menu">
    menu Button
  </button>

  <div class="menu_about">
    Menu
  </div>
</div>