So at this point I think we should do outreach to Gmail folks to let them know about this issue and see if they can improve their usability to avoid this scrollbar overlap. And we should let them know that we have the ability to let ::-webkit-scrollbar force legacy scrollbars, and if they want to rely on that (as they seem to be sort-of doing), we'd like them to send us those styles, modulo bug 2057977.
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So at this point I think we should do outreach to Gmail folks to let them know about this issue and see if they can improve their usability to avoid this scrollbar overlap. And we should let them know that we have the ability to let ::-webkit-scrollbar force legacy scrollbars, and if they want to rely on that as part of mitigating the usability issues (as they seem to be sort-of doing), we'd like them to send us those styles, modulo bug 2057977.