> Chrome have fortunately recently released a "extension side panel" mode, and since only DOM changes can be easily identified, using the chrome extension side panel would be virtually un-detectable however this is far less intuitive to use and requires the user to perform some action to open the sidepanel every time they want to use the extension.

As an end user I could not find an option to open the side panel

Yeah I mean it's not very commonly used by extensions. I quite like it as it's completely isolated and not detectable. I built my first extension which uses it as the primary interface yesterday: https://github.com/Am-I-Being-Pwned/PGP-Tools