From https://codereview.chromium.org/2371933003/#msg10:

... the pixel snapping in documentRect() looks incorrect. The call sites to it
should I think be adjusted to flow
subpixel positioning through and snap at the end. e.g.:

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/co...

is not adding up the subpixelAccumulation before snapping.