I understand this is an old thread, but I have a related problem where the TabsOutliner window width is too wide because Vivaldi windows won't shrink below 600px wide. I know tabsoutliner can be loaded in the panel but I'd like to use the panel on the right side with tabsoutliner on the left as a narrow window. Currently I can't get this working.
And before you suggest just dropping tabs outliner and using vivaldi's built in vertical tabs/window manager, I suggest you give tabsoutliner a try for a few weeks to understand the functionality it offers.
Are there any ways to run a smaller/headless vivaldi window or something? Currently when opening TabsOutliner there is an address bar and everything while in chrome and edge for example, those are gone and TabsOutliner has its own minimal window that can be resized very narrow.