Content inside a non-stacking context overflow:scroll can overlap overlay scrollbars.
Created attachment 408294 [details]
Created attachment 408307 [details]
Created attachment 408359 [details]
Created attachment 408409 [details]
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