[Webkit-unassigned] [Bug 270358] New: IntersectionObserver does not fire on a headless linux environment

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=270358

            Bug ID: 270358
           Summary: IntersectionObserver does not fire on a headless linux
                    environment
           Product: WebKit
           Version: Safari 17
          Hardware: Unspecified
                OS: Linux
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ldebeasi at gmail.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 470116

  --> https://bugs.webkit.org/attachment.cgi?id=470116&action=review

Reproduction

When testing with WebKit 17.4 on Linux, IntersectionObserver callbacks do not reliably fire in a headless environment. Running the same tests in a headed environment do not reproduce the issue.

Steps to reproduce:

1. Open attached reproduction in WebKit 17.4 on Linux in a headless environment.
2. Observe that the page logs "Has .test-ready class? true". This class should be added once the div is visible.
3. Wait 1000ms. Observe that the page logs "Has .test-ready class after hiding? false". This should be removed because the div is hidden.
4. Wait 1000ms. Observe that the page logs "Has .test-ready class after re-showing? false". This is incorrect as the div is visible again.

Expected Behavior:

I expect the "test-ready" class to be added when the element is made visible again in step 4.

Actual Behavior:

The "test-ready" class is not added when the element is made visible again in step 4.

Other information:

- The inline styles for the "body" seem to be influencing this behavior too, though I can't quite understand why.
- This does not happen in headless Chrome or Firefox browsers.

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