Animation issue on Firefox Quantum

Question

I noticed an issue with animations on the new Firefox Quantum.

When you first load a page with some animated elements display: none;, when a script switches it to .display = "block"; you will miss the entire animation, or some parts of it at the beginning if it is longer than a few seconds.

View it in the snippet below:

var anims = document.getElementsByClassName("anim"),
    time = document.getElementById("time"),
    interval = null;

function animate() {
  for (var i = 0; i < anims.length; i++)
    anims[i].style.display = "block";
}

function timer(sec) {
  time.textContent = sec--;
  interval = setInterval(function () {
    time.textContent = sec >= 0 ? sec-- : clearInterval(interval) || "";
  }, 1000);
}

// Directly after click
button0.addEventListener("click", animate);

// One seconds after click
button1.addEventListener("click", function () {
  timer(1);
  setTimeout(animate, 1000);
});

// Two seconds after click
button2.addEventListener("click", function () {
  timer(2);
  setTimeout(animate, 2000);
});

// Three seconds after click
button3.addEventListener("click", function () {
  timer(3);
  setTimeout(animate, 3000);
});

// Hide the divs
reset.addEventListener("click", function () {
  for (var i = 0; i < anims.length; i++)
    anims[i].style.display = "none";
});

body {
  font-family: arial;
}

body > div {
  margin-bottom: 10px;
}

#result {
  background-color: #e5f3ff;
  height: 120px;
  padding: 10px;
}

.anim {
  display: none;
  float: left;
  margin: 10px;
  width: 50px;
  height: 50px;
  border-radius: 5px;
  animation: animate 1.5s;
}

#anim1 {
  background-image: linear-gradient(120deg, #a1c4fd 0%, #c2e9fb 100%);
  
  /* Only one iteration iteration (default) */
  /* This one will not be animated */
}

#anim2 {
  background-color: #fddb92;
  animation-iteration-count: 3; 
  
  /* Three iterations */
  /* Only one iteration will be seen */
}

#anim3 {
  background-image: linear-gradient(45deg, #ff9a9e 0%, #fad0c4 99%, #fad0c4 100%);
  animation-iteration-count: infinite; 
  
  /* Infinite */
  /* No visible problem */
}

@keyframes animate {
  50% {
    transform: translate(80%, 100%) rotate(-360deg);
  }
}

<div>
  <span><strong>Reload the snippet</strong>
  before clicking another button for viewing the issue
  <br/><strong>Or,</strong>
  <em>Reset</em> (display: "none") before clicking a button to view with no issue: </span>
</div>

<div>
  <button id="button0">On click</button>
  <button id="button1">1 sec timeout</button>
  <button id="button2">2 sec timeout</button>
  <button id="button3">3 sec timeout</button>
  <button id="reset">Reset</button>
  <span id="time"></span>
</div>

<div id="result">
  <div id="anim1" class="anim"></div>
  <div id="anim2" class="anim"></div>
  <div id="anim3" class="anim"></div>
</div>

You will notice that the infinite animation doesn't apparently have any problem, but the two others do obviously have.

What is the solution then?

Note:

• You have to use Firefox Quantum in order to view this.
• I have tried the same snippet on Google Chrome and everything is working good.

Answer 1

Tested it, pretty sure it is solved for all browsers by using classes. There are more ways to handle it but putting the animation inside a new class that only gets added after the button click does the trick.

In the CSS I've moved the animation property to a new class, and the new class also add the block style.

.anim-start {
  display: block;
  animation: animate 1.5s;
}

In the JS I only changed style.display='block' to anims[i].classList.add('anim-start');

See: https://jsfiddle.net/0mgqd2ko/1/

Using this method of a new class makes it easier. For example, what if you want to transition from opacity 0 to 1? It's hard to do that when starting from display none. And what if you just want to use visibility so the elements still take space?