[Webkit-unassigned] [Bug 160953] New: WebKit incorrectly clips position:fixed element, inside of its "position:relative; overflow: hidden" parent, IF that parent has "z-index" set

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=160953

            Bug ID: 160953
           Summary: WebKit incorrectly clips position:fixed element,
                    inside of its "position:relative; overflow: hidden"
                    parent, IF that parent has "z-index" set
    Classification: Unclassified
           Product: WebKit
           Version: Safari 9
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: dholbert at mozilla.com
                CC: simon.fraser at apple.com

STR:
 1. Load this testcase: https://jsfiddle.net/bo4q2hbh/

EXPECTED RESULTS:
 Teal div should not be clipped at orange border. (The teal div is a fixed-position element, and it should use the viewport as its containing block, rather than using its orange-bordered relpos ancestor.)

ACTUAL RESULTS:
 Teal div is clipped at orange border! (in Safari)

I get EXPECTED RESULTS in Firefox 48, Edge 14, and Chrome 54.
I get ACTUAL RESULTS in Safari 9.1 (on Mac OS El Capitan)

The bug only happens if the relatively-positioned ancestor has "z-index" set.  In my above-linked jsfiddle, that element has "z-index: 0". If I remove that line, the bug goes away and I end up with EXPECTED RESULTS.

Note: Chrome has a similar bug, but their bug is more restricted (it only happens if there's also 3d-transformed descendant).
https://bugs.chromium.org/p/chromium/issues/detail?id=638780

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