I certainly won't argue that there are a lot of feature requests, however, the code to implement this already exists in the Vivaldi source code since it's Chromium based.
I know enough C++ to read (aka fumble) through code, but not enough to write anything, otherwise I would give it a go. Hopefully I am on track here and a Vivaldi dev sees this and can check this request off the list easily. 🙂
I was using the source for Vivaldi version 6.8.3381 for my detective work.
See the file <vivaldi-source-folder>/chromium/chrome/common/chrome_switches.cc, and search it for --window-name. That is the Chromium flag to set the window title; I use it all the time from a rofi script to launch Chrome with different profiles and window names.
I did a little more poking and found a function in <vivaldi-source-folder>/base/vivaldi_browser_window.cc. That function is VivaldiBrowserWindow::GetTitle(), which I believe is getting the page title and then appending the string - Vivaldi and returning the whole string.
Just above that is a function VivaldiBrowserWindow::UpdateTitleBar() that is referencing a function by the name of UpdateWindowTitle(). I believe that is the function that needs to be changed, but that appears quite a few times in the source. I think it's origin is in <vivaldi-source-folder>/chromium/ui/views/bubble/bubble_frame_view.cc, but I could be wrong. Regardless of where it is, to mimic Chromium, it would be a simple if/else. Something like:
# Pseduo code:
if --window-name value != ''
window title == --window-name value
else
window title == page title + " - Vivaldi"
Anyway, I'm just trying to be helpful as I would like this feature too!
Thanks!