Mail provider Fastmail.fm which is programmed by former Opera employees, does not work properly with Vivaldi. There is a preference pane called Settings which does not appear with Vivaldi but it does appear in other browsers. Within it is a panel called Theme, which therefore does not function. The there permits a sidebar which lists folders. Under Vivaldi, no folders unless you press a left arrow. I reported this to Fastmail support, but they have no experience with Vivaldi at this point. So, please fix if possible. Like many people, the only other thing holding me back from using Vivaldi regularly is extension, so I await the further progress of the app.
I should add that this is not an issue for Windows 7. Just for Yosemite.
Please, while waiting for replies of users with experience on fastmail
file a bug report
Thank you, have now filed a report
I'm happy to report that this is fixed with 22.214.171.124
Thank you very much.