e.g. if an application overrides the `-underPageBackgroundColor` with a dark color on a page that has a light background color, the scroll bar will appear light to "match" the overridden `-underPageBackgroundColor` instead of dark to "match" the page's background color
<rdar://problem/80116822>
Created attachment 434150 [details] Patch
Created attachment 434151 [details] Patch
Committed r280283 (239941@main): <https://commits.webkit.org/239941@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 434151 [details].