Content inside a non-stacking context overflow:scroll can overlap overlay scrollbars.
Created attachment 408294 [details] Patch
rdar://problem/61918702
Created attachment 408307 [details] Patch
Created attachment 408359 [details] Patch
Created attachment 408409 [details] Patch
Committed r266825: <https://trac.webkit.org/changeset/266825> All reviewed patches have been landed. Closing bug and clearing flags on attachment 408409 [details].
Logging fix in https://trac.webkit.org/changeset/266884/webkit