[Webkit-unassigned] [Bug 224953] New: Elements with negative margin in scrollable regions no longer hidden by parent element padding

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=224953

            Bug ID: 224953
           Summary: Elements with negative margin in scrollable regions no
                    longer hidden by parent element padding
           Product: WebKit
           Version: Safari 14
          Hardware: Mac (Intel)
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: bugs.webkit at johnwhelchel.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 426856

  --> https://bugs.webkit.org/attachment.cgi?id=426856&action=review

Safari 14 (Bad)

A recent bug was introduced by Safari 14 where elements positioned via negative margin to be hidden "behind" the padding of a parent are no longer hidden if they are contained within a Scrollable area.

I'm not a layout expert, so this is my best interpretation of what is happening.

However, this JSFiddle reproduces the issue in a roughly minimal way: https://jsfiddle.net/whelchel/veL7dkyj/18/. I've also attached one screenshot - one from Safari 14 (bad). I will try and see if I can attach a second with the good behavior.

In Safari 13, both times the SVG positioned with negative margin is hidden. This is the correct, expected behavior.

In Safari 14, when contained by a scrollable, the SVG is visible, despite the padding that should "cover" it. You can compare the top and bottom instances to see the difference, with and without the scrollable.

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