[Webkit-unassigned] [Bug 79949] New: Semi-transparent image translated over a negative-scaled image produces painting artifacts

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=79949

           Summary: Semi-transparent image translated over a
                    negative-scaled image produces painting artifacts
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
               URL: http://code.google.com/p/chromium/issues/detail?id=109
                    067
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: tabatkins at google.com


Steps to reproduce:
1. Create a page with two images overlaid atop each other.  The upper image must be semi-transparent; the lower image doesn't matter.
2. Transform the lower image with a negative-scale transform, like "scale(-1)".
3. Translate the upper image across the lower image, with an animation or manually.

Effect: As the upper image is translated across the lower image, it leaves behind "trails" of pixels on the lower image.

This bug only appears to happen when the lower image is transformed as described.  Any other transform doesn't trigger the issue.

Reduced testcase and screenshot of problem located at the original bug in the Chromium bug tracker: <http://code.google.com/p/chromium/issues/detail?id=109067>

The Chromium bug states that this bug happens on Windows, but not Mac.  I can confirm that it also occurs on Linux.

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