[Webkit-unassigned] [Bug 247862] New: Rendering bug when using position:fixed with containing block

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=247862

            Bug ID: 247862
           Summary: Rendering bug when using position:fixed with
                    containing block
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: webkit at game-point.net
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

For an example, see: https://game-point.net/misc/stickyMaskOverScroll/index4.htm

When a div is created with 'position:fixed' inside a container div with 'transform:translate(0); overflow:auto; max-height:300px;', and it is then given a sibling div whose content causes the container to overflow, the 'fixed' div should not change its position when the overflowing content is scrolled, in the same way that it doesn't if the fixed positioning containing block is the viewport.  However, when 'transform:translate(0)' is applied to the container to establish it as the fixed positioning containing block, the 'fixed' div starts to incorrectly become scrolled along with the overflowing content as if its positioning were set to 'absolute'.

Relevant part of the CSS spec: https://w3c.github.io/csswg-drafts/css-position/#fixed-cb

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