Add a way to show the scrollbar in the tab list:

1. Open the /opt/vivaldi/resources/vivaldi/bundle.js file
2. Find the following string:

flexDirection:t,width:a

3. Add 20000 or any other value:

flexDirection:t,width:a+20000

4. Import the vivaldi_ui.css file
5. Add the following string to browser.html:

<script src="vivaldi_ui.js"></script>

6. Copy the vivaldi_ui.js file to the /opt/vivaldi/resources/vivaldi folder.
7. Restart the browser to see the changes!

Tips for debugging Vivaldi:

Run Vivaldi with the debugging port:

vivaldi --remote-debugging-port=32167

Open localhost:32167, find the window and use the dev tools to hack Vivaldi!