[Webkit-unassigned] [Bug 173466] New: Layer compositing under scaling or rotation bleeds fully obscured layers

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=173466

            Bug ID: 173466
           Summary: Layer compositing under scaling or rotation bleeds
                    fully obscured layers
           Product: WebKit
           Version: Safari 10
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: onne at framer.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Two layers with identical sizes and transforms and placing, the top layer should fully obscure the first. But instead, depending on scaling or rotations, there is horrible bleeding. Firefox works except under rotation. 3d rendering api's work this way.

example html:
```
<!--
Demonstration of bleeding in layer composition.
1) Remove scale3d() from the outer div, or using "nicer" scales, and there is no bleeding.
2) Setting rotateZ(1deg) on the inner divs and there is also bleeding.
(Notice mozilla only has bleeding for case 2.)
-->
<div id="outer" style="
    position: absolute;
    width: 1000px;
    height: 1000px;
    transform: translate3d(50px, 50px, 0px) scale3d(0.61, 0.61, 1);
    background-color: rgba(255, 220, 220, 1);
">
    <div id="inner1" style="
        position: absolute;
        width: 400px;
        height: 500px;
        transform: translate3d(10px, 10px, 0px) rotateZ(0deg);
        background-color: rgb(0, 0, 0);
        "></div>
    <div id="inner2" style="
        position: absolute;
        width: 400px;
        height: 500px;
        transform: translate3d(10px, 10px, 0px) rotateZ(0deg);
        background-color: rgba(220, 255, 220, 1);
        "></div>
</div>
```

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