[Webkit-unassigned] [Bug 173913] New: Overflow percentage widths calculated incorrectly

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=173913

            Bug ID: 173913
           Summary: Overflow percentage widths calculated incorrectly
           Product: WebKit
           Version: Safari 10
          Hardware: iPhone / iPad
                OS: iOS 10.3
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: rez at iinet.net.au

Created attachment 314000

  --> https://bugs.webkit.org/attachment.cgi?id=314000&action=review

Correct behaviour in MacOS

Steps to reproduce the problem:
Over-sized elements calculate percentages based on when overflow is applied.

See https://codepen.io/cliener/pen/PjjJYM?editors=1100#0

What is the expected behavior?
Percentage widths should be calculated on the width of the parent instead of the element itself.

For example, if the window width is 1000px and the over-sized element 1400px, setting width of 50% on the over-sized element should set the width to 500px.

What went wrong?
Widths are calculated on the width of the element not of the parent.

Following the example above, if the window width is 1000px and the over-sized element 1400px, setting width of 50% on the over-sized element sets the width to 700px.

Did this work before?
Safari 10.1.1 MacOS calculates the value correctly. Safari in iOS 10.3.2 does not. Unsure about earlier versions.

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