Created attachment 334159 [details]
Support for root background was added in bug 176261. However, it seems that this does not take into account shadow on the root. See the attached testcase: The orange shadow is not visible when the yellow background is present on the root.
Actually, this seems unrelated to Async frame scrolling, it already happens in Safari macOS / iOS when the setting is disabled. Gecko and Chromium render the box shadow even when the background is present.