[Webkit-unassigned] [Bug 226739] New: iFramed position:fixed element doesn't show in viewport after page regains focus

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=226739

            Bug ID: 226739
           Summary: iFramed position:fixed element doesn't show in
                    viewport after page regains focus
           Product: WebKit
           Version: Safari 14
          Hardware: Mac (Intel)
                OS: macOS 10.15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Compositing
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: kdolan at g.harvard.edu
                CC: simon.fraser at apple.com

When an iFramed document contains a position:fixed element inside the scrollable body. The position fixed element moves along with the viewport fine when the page loads and has focus. But, tab away to another tab, then back to the page, and, in Safari, the position:fixed element is hidden at the top of the page. Not at the scrolled position. It's as if the container box is not the viewport anymore but the body.

However, in Safari inspector, the position:fixed element is registering as existing in the viewport area. The position:fixed element's bounding box is in the fixed:position where the element should be visible. But, the position:fixed element is not visible there, its hidden at the top. The element only snaps into the viewport when the body is manually scrolled. Attempting to scroll the body with javascript does not show the position:fixed item in the viewport. This is only an issue in Safari. Firefox and Chrome show the item in its fixed position fine, with no problems, after the page gets focus again.

Example: https://jsfiddle.net/khdolan/o1md489z
In Safari, tab to another page and back to the rendered fiddle to see the issue.

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