[Webkit-unassigned] [Bug 115304] New: REGRESSION(SUBPIXEL_LAYOUT) Composited layers can cause one pixel shifts

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=115304

           Summary: REGRESSION(SUBPIXEL_LAYOUT) Composited layers can
                    cause one pixel shifts
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: allan.jensen at digia.com
                CC: zalan at apple.com
            Blocks: 94792,113199


The issues was observed on hulu.com, the reduced test case is one div on a subpixel position with a an image below it also on a subpixel position. If the outer div is caused to become composited the image below it may change its pixel rounding, which causes it to shift.

Of the issues one pixel offset observed by Mac, this is the only one reliably observed on Qt as well.

Imagine element A a placed at (0,  10.6) with an image B inside placed relatively to A at (0, 4.8). The image B then has an absolute position of  (0, 15.4) which causes it to rendered at (0,15). If A becomes accelerated composited, then A will be rendered at (0,5) on the composited layer, and the composited layer will be rendered at (0,11), causing image B to end up at (0,16).

To avoid this issue, the position of accelerated layers must be floored to an integer position, and be given an internal subpixel padding of the remainder, making their content get the at the same fractional coordinates as if there were no compositing.

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