EventHandler::mouseMoved doesn't seem to do the right thing when finding overflow scrollers in iframes, which breaks overlay scrollbar interaction.
<rdar://problem/62784560>
Created attachment 398469 [details] Patch
Created attachment 398824 [details] Patch
Created attachment 398825 [details] Patch
Created attachment 398832 [details] Patch
Created attachment 398837 [details] Patch
Committed r261382: <https://trac.webkit.org/changeset/261382> All reviewed patches have been landed. Closing bug and clearing flags on attachment 398837 [details].
Added missing tests in https://trac.webkit.org/changeset/261426/webkit