[Webkit-unassigned] [Bug 175033] New: Incorrect border clipping caused by border-radius on container element w/ fractional pixel width (Retina only)

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=175033

            Bug ID: 175033
           Summary: Incorrect border clipping caused by border-radius on
                    container element w/ fractional pixel width (Retina
                    only)
           Product: WebKit
           Version: Safari 10
          Hardware: Macintosh
                OS: macOS 10.12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: thecoreh at gmail.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 316867

  --> https://bugs.webkit.org/attachment.cgi?id=316867&action=review

Test case that reproduces the bug

On Retina Displays, Webkit is incorrectly clipping the right border of a nested element when its parent:

1- Has `border-radius` set to a value other than 0;
2- Has a non-integer computed pixel width (Caused, for instance by setting its width to something like 33.3333%, and resizing the window to a width that's not a multiple of 3)

I can reproduce the error on both Safari 10 and on the Nightly Webkit builds. I've found other bugs related to clipping, but couldn't find one with the exact repro steps of this one, so sorry if it's a duplicate.

I've attached a testcase that triggers the bug.

Instructions:

1. Open testcase.html on Safari on a Mac with a Retina display. 
2. The page should render a rectangle with a red 1px border (= 2 device pixels on the retina display).
3. Slowly resize the Safari window horizontally
4. For some window sizes, the right border of the rectangle will get incorrectly clipped to 0.5px (= 1 device pixel on the retina display)

Thanks.

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