[Webkit-unassigned] [Bug 219495] New: IntersectionObserver {root:document}: Getting a different rootBounds.top than both FF/Chrome for a scrolled window

Thu Dec 3 09:56:04 PST 2020

https://bugs.webkit.org/show_bug.cgi?id=219495

            Bug ID: 219495
           Summary: IntersectionObserver {root:document}: Getting a
                    different rootBounds.top than both FF/Chrome for a
                    scrolled window
           Product: WebKit
           Version: Safari 14
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: New Bugs
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: friedj at google.com

Hi webkit folks!

While working on a PR to update the w3c/IntersectionObserver polyfill with `root:document` support, I believe I may have found an issue with the WebKit implementation.

A `root:document` IntersectionObserver seems to be firing entries with a `rootBounds` that is relative to the scrollingElement.scrollTop. Both Chrome and FF always use `top:0` in this case.
According to the spec, I think the rootBounds should be equal to the document viewport (spec: https://www.w3.org/TR/intersection-observer/#intersectionobserver-root-intersection-rectangle).

Example:
```html
<!DOCTYPE html>
<html>
<head><title>IntersectionObserver RootBounds Test.</title></head>
<body>  
  <script>
    function init() {
      window.document.body.style = 'height: 2000px';
      window.scrollTo(0, 110);
      const io = new window.IntersectionObserver((e) => {
        const p = document.createElement('p');
        p.innerText = `rootBounds: ${JSON.stringify(e[0].rootBounds)}`;
        document.body.appendChild(p);
      }, {root: document});

      io.observe(document.querySelector('body'));
    }
    window.addEventListener('DOMContentLoaded', init);
  </script>
</body>
</html>
```

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