[Webkit-unassigned] [Bug 139377] New: Half-pixel margin causes incorrect border-bottom rendering

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=139377

            Bug ID: 139377
           Summary: Half-pixel margin causes incorrect border-bottom
                    rendering
    Classification: Unclassified
           Product: WebKit
           Version: 528+ (Nightly build)
          Hardware: Macintosh Intel
               URL: http://buzzera.com/border-bottom-rendering.html
                OS: Mac OS X 10.9
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: New Bugs
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jonathan at buzzera.com

Created attachment 242773
  --> https://bugs.webkit.org/attachment.cgi?id=242773&action=review
Reduced test case

With some specific CSS rules, the border-bottom of an element is rendered too light or not at all. See the attached test page and screenshot.

Requirements to trigger:
* margin-top: half-pixel value (i.e., 10.5px, or 0.65625em which is 10.5px if 1em=16px)
* border: 1px solid with some color
* border-[left/right]-width: 0px
* border-bottom-[left/right]-radius: >= 2px

Two elements with these rules should be placed after each other. Depending on the elements above it, one of these will have the rendering issue:

If one of border-bottom-[left/right]-radius is set, the border-bottom is rendered only half visible; if both are set, the border-bottom is not rendered at all.

The issue might seem far-fetched, but I encountered it in a real project I'm working on. Especially when using em's for margins you can't foresee half-pixel margin values, and thus trigger this issue.

Tested in:
* Safari 7.1 (9537.85.10.17.1) with WebKit 600.1.17
* Nightly: Safari 7.1 (9537.85.10.17.1, r176637) with WebKit 601.1.11+

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