Bug 240653

Summary: [iOS] opacity animation changes computed font-size
Product: WebKit Reporter: Mauricio <mauricio>
Component: AnimationsAssignee: Nobody <webkit-unassigned>
Status: NEW ---    
Severity: Critical CC: dino, graouts, graouts, mmaxfield, webkit-bug-importer, zalan
Priority: P2 Keywords: HTML5, InRadar
Version: Safari 15   
Hardware: iPhone / iPad   
OS: iOS 15   

Description Mauricio 2022-05-19 08:10:42 PDT 
Safari iOS has a bug that will make a `font-size` property scale up consistently in the presence of an opacity animation. The computed value diverges from the set CSS value. But even without a set CSS value, you can see how the text jumps, speak the `font-size` grows at the end of the animation.

To reproduce the bug, you need a parent DOM element containing a unit width or height + `overflow: hidden. At the same time, you require at least two `<p>`. And last a DOM element with `opacity: 0` will be set to 1 through JavaScript (See the HTML example below).

You can see a live demo in this CodeSandbox. https://8d838r.csb.app/

This is a minimal representation of the bug.

<!DOCTYPE html>
<head>
  <style>
    .container {
      overflow: hidden;
      width: 100%;
      height: 2000px;
      background: yellow;
    }

    .transition {
      opacity: 0;
      background: red;
      width: 9px;
      height: 9px;
      will-change: transform;
      z-index: 2;
    }
  </style>
</head>
<body>
  <div class="container">
    <p>XXX</p>
    <p>XXX</p>
    <p>XXX</p>
    <p>XXX</p>
    <div id="ani" class="transition"></div>
  </div>

  <script>
    setTimeout(() => {
      document.getElementById("ani").style.opacity = 1;
    }, 2000);
  </script>
</body>
Comment 1 Sam Sneddon [:gsnedders] 2022-05-19 08:45:30 PDT 
I feel like I've seen this bug recently, but I can't find any duplicate, Antoine?
Comment 2 Antoine Quint 2022-05-19 08:47:34 PDT 
Does not ring a bell!
Comment 3 Radar WebKit Bug Importer 2022-05-26 08:11:12 PDT 
<rdar://problem/93970773>