position:absolute element messes up fading of another

Question

I'm trying to fade a text but it just pops to the end states without fading in-between (partially with delay, too).

I'm using CSS for the fading (opacity: 0/1 and transform: opacity) and, after a lot of debugging I discovered that this was happening due to a totally unrelated position: absolute element underneath. Here's a fiddle:

var $root = $('.root');
$('#butt_fadetext').click(function(){
  $root.toggleClass('dermineyda');
});
$('#butt_hidefancy').click(function(){
  $root.toggleClass('nofancy');
});

.root {
  background-color: #b87988;
}

.container {
  transition: opacity 0.5s;
  opacity: 1;
  height: 100px;
}

.dermineyda .container {
  opacity: 0;
}

.inn {
  position: absolute;
  z-index: 1;
}

.unrelated {
  background-color: blanchedalmond;
  width: 100px;
  height: 30px;
  position: absolute;
  top: 0;
  left: 0;
}

.nofancy .unrelated {
  display: none;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<div class="root">
  <div class="container">
    <div class="inn">text</div>
  </div>
  <div class="unrelated">fancy fx</div>
</div>
<input type="button" value="fade text" id="butt_fadetext" />
<input type="button" value="hide fancy fx" id="butt_hidefancy" />

Answer 1

I learned just now that changing an element's opacity can alter its order in the stacking context:

Using this property with a value different than 1 places the element in a new stacking context.

Solution

I fixed it by forcing its order upwards in the stacking context.

var $root = $('.root');
$('#butt_fadetext').click(function(){
  $root.toggleClass('dermineyda');
});
$('#butt_hidefancy').click(function(){
  $root.toggleClass('nofancy');
});

.root {
  background-color: #b87988;
}

.container {
  transition: opacity 0.5s;
  opacity: 1;
  height: 100px;
  position: relative;
  z-index: 500;
}

.dermineyda .container {
  opacity: 0;
}

.inn {
  position: absolute;
  z-index: 1;
}

.unrelated {
  background-color: blanchedalmond;
  width: 100px;
  height: 30px;
  position: absolute;
  top: 0;
  left: 0;
}

.nofancy .unrelated {
  display: none;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<div class="root">
  <div class="container">
    <div class="inn">text</div>
  </div>
  <div class="unrelated">fancy fx</div>
</div>
<input type="button" value="fade text" id="butt_fadetext" />
<input type="button" value="hide fancy fx" id="butt_hidefancy" />