title: Intersection Observer doesn't works properly when a CSS Transform brings the element inside viewport
body: <p>I initialise an Intersection Observer without specific options so the threshold equals to 0.0 and the event should trigger just when the first pixel of the observed element enters the viewport.</p>
  
  <p>That's what happens when I trigger the observer by scrolling and also when I trigger it using a transition defined with a simple CSS property like <code>top</code>. But when the observed element appears in the viewport thanks to a CSS transform that is animated with a transition the observer doesn't triggers the callback until the animation is over. <em>I need it triggered just in the moment the element appears inside the viewport, as it should be</em>.</p>
  
  <p>You can see an example here: <a href="https://jsfiddle.net/38v2dots/2/" rel="noreferrer">https://jsfiddle.net/38v2dots/2/</a></p>
  
  <p>My real world problem is with a carrousel library that works (unnecessarily) with a CSS 3D transform. But the problem happens also with 2D transforms. Unfortunately I'm forced to use this library so <em>avoiding the 3D transform isn't an option</em>.</p>
  
  <p>Thank you in advance.</p>