Version: 51.0.2704.106 (Official Build) (64-bit)
OS: Linux

What steps will reproduce the problem?
(1) Load the attached file or http://output.jsbin.com/xoyagud

What is the expected output?
The background should be gray (50% black on top of white).

What do you see instead?
Instead the background is solid black. This is happening because the check in CompositedLayerMapping for whether the background is opaque[1] thinks that the background clip is border-box (i.e. the opaque background color should be drawn under the border) but when you set background attachment local the background is clipped to the content box. See

[1] https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp?sq=package:chromium&dr=CSs&l=328 
[2] "Because the scrollable area does not include the border area, for scrollable elements the ‘border-box’ value of ‘background-clip’ may be treated the same as ‘padding-box’." https://www.w3.org/TR/css3-background/#the-background-attachment
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Deleted: local-background.html 214 bytes

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