This seems to fix it for me. It appears we calculate the "hypothetical position" correctly but we fail to translate into the right coordinate space. Oriol, does the WPT in this patch pass in Edge?
Bug 1520584 Comment 2 Edit History
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This seems to fix it for me. It appears we calculate the "hypothetical position" correctly but we fail to translate it into the right coordinate space. Oriol, does the WPT in this patch pass in Edge?