[Webkit-unassigned] [Bug 240653] New: [iOS] opacity animation changes computed font-size

Thu May 19 08:10:42 PDT 2022

https://bugs.webkit.org/show_bug.cgi?id=240653

            Bug ID: 240653
           Summary: [iOS] opacity animation changes computed font-size
           Product: WebKit
           Version: Safari 15
          Hardware: iPhone / iPad
                OS: iOS 15
            Status: NEW
          Severity: Critical
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: mauricio at framer.com

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>

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