[Webkit-unassigned] [Bug 176384] New: Fixed-position block within fixed-position, overflow-scrolling block is unexpectedly clipped

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=176384

            Bug ID: 176384
           Summary: Fixed-position block within fixed-position,
                    overflow-scrolling block is unexpectedly clipped
           Product: WebKit
           Version: Safari 10
          Hardware: Macintosh
                OS: macOS 10.12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: evan at artefactgroup.com

Created attachment 319917

  --> https://bugs.webkit.org/attachment.cgi?id=319917&action=review

Initial test case demonstrating the described issue

OVERVIEW

- Given a fixed-position block (A) with `overflow: scroll`
- And a second, fixed-position block (B) within it
- When block A is positioned to fill the entire right half of the viewport
- And block B is positioned to occupy the center of the viewport
- Then the left half of block B is unexpectedly clipped by block A

STEPS TO REPRODUCE

- Download attached test case (test-case-01.html).
- Open test case in Safari.

EXPECTED RESULTS

- Modal dialog is fully visible.

ACTUAL RESULTS

- Left half of modal dialog is clipped.

BROWSERS TESTED

- Safari 10.1.2, macOS 10.12.6: issue occurs
- Safari nightly 10.1.2 (12603.3.8, r221610), macOS 10.12.6: issue occurs
- Firefox 55.0.3, macOS 10.12.6: no trouble found
- Chrome 60.0.3112.113, macOS 10.12.6: no trouble found

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