[Webkit-unassigned] [Bug 118624] New: Fix border radius glitch

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=118624

           Summary: Fix border radius glitch
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Keywords: BlinkMergeCandidate
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: rniwa at webkit.org
                CC: simon.fraser at apple.com, dino at apple.com,
                    thorton at apple.com


Consider merging https://chromium.googlesource.com/chromium/blink/+/927872e324ae7520ab32567d07fb988b1733bec8

This fixes the bug that the border radius partially disappeared or had wrong color when it was too steep or too shallow.

Fix 1: Use the correct clipping quad, generated by intersecting border mitre lines to diagonal lines of the corner ellipse arc.
RenderBoxModelObject::clipBorderSidePolygon was generating wrong clipping quad.
It was based on a wrong assumption that rounded border would not cross {horizontal,vertical} line through the box center.

Fix 2: Correctly split a quad into two quads.
A clipping quad is split into "AND" of two quads to set anti-aliasing on both edges individually.
Generate these quads properly by extending a vertex parallel to the opposite edge.

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