Taskbar affected when Vivaldi maximised
A known issue that I reported before. I think it happens after using Vivaldi in Fullscreen, not just maximised.
(VB-17670) Vivaldi Breaks Autohide of the Windows Taskbar
Just disable the native windows mode, until the bug will be fixed
Disabling Use Native Window doesn't help and it has nothing to do with fullscreen. It's just a gremlin from the past, which reappeared in 1.2.