Most major browsers offer an "app mode", which allows the application to be started on a single web page without typical browser UI like tabs, panels and address bar.
Vivaldi has a similar feature which is used when it is started with the "--app=url" command line option. I think this is also used when opening some pop-ups. However, this mode still shows a modified address bar, which makes it unsuitable for "app mode". I would like to request an option to hide the address bar in this popup/app mode.
(It is currently only possible to achieve this using custom css code.)
Alternatively, Vivaldi achieves the same effect with the "toggle UI" (ctrl+F11). It would be good if we could start Vivaldi without the UI using a command line option.
Don't know if it's still relevant but look at this thread https://forum.vivaldi.net/topic/34129/start-in-application-mode-without-ui-from-command-line/7