I'm trying to switch to Wayland with 6.5. What I noticed is that Plasma now closes Vivaldi windows one by one, as opposed to close the entire application. From the browser perspective it seems like the user manually closed these windows. The result: the next time I open the browser, only the last closed window gets reopened, potentially losing hundreds of tabs (I know...) from the other windows. Since they were "manually" closed, Vivaldi won't reopen them. On X11 it worked well. Vivaldi application, as a whole, was closed during reboot and all of its windows were restored when I re-launched the application.