[Webkit-unassigned] [Bug 156817] New: position:fixed and background-color lead to painting artifacts on non-retina displays

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=156817

            Bug ID: 156817
           Summary: position:fixed and background-color lead to painting
                    artifacts on non-retina displays
    Classification: Unclassified
           Product: WebKit
           Version: Safari 9
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: kneebraces22 at gmail.com
                CC: simon.fraser at apple.com

This bug is a little finicky and doesn't manifest consistently. However, the setup in this JSFiddle (https://jsfiddle.net/Lcu6Lrv9/7/) seems to reproduce it fairly reliably. 

We have three dom elements: a container which is styled with 'position: fixed', a box with a background color, and a div with a background image that is scaled / positioned to be centered within our box. Often the background color of the box element that is *outside* of the bounding box of the image is colored incorrectly. The incorrect color seems to be a function of the original background color (e.g if our background color is blue the buggy color is orange). I've reproduced this using a background-image, an <img> tag, and with an inline SVG (though it seems to happen most consistently with the setup above.) I have also reproduced this bug with PNGs and SVGs so I don't think it's related to the image type. The most important aspect seems to be that the top level container element has 'positioned: fixed'.

This bug *only* manifests on non-retina displays. I have never reproduced it on a retina display. Usually opening the inspector or dragging the window around or anything that might cause a repaint causes the weird background color to fix itself.

The only workaround I have found so far is to use a pre-scaled image that is larger that the box element with transparent background, rather than scaling the image with CSS.

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