Only workaround to have some glass, is to use "native window" under appearence which can use the DWM composition. But no glass behind tabs, as they are outside any WinAPI: they are part of Vivaldi UI, actually. DWM glass API (sad as I love it) is cosidered obsolete/deprecated in several software(office2013+)/OS(win8+) - which is fun, as several apps still use that if their code allow to use that. It's a "cool useless function" which actually divide the win users xD