It's been ~18 months so I'm fuzzy on details. I remember gmail being tricky also.

IIRC I ended up building an iframe based scraper for sites that didn't yield any content with just a fetch - and I think built a fallback mechanism so that if fetch didn't work, I'd queue it up in the iframe scraper. The problem with that is that there are various heavily used security headers that prohibit loading in an iframe. (And the reason for iframe vs just loading in a tab and injecting my extension's script is that I wanted it to be able to run "in the background" without being super distracting for the user - the tab changing favicon every second or two was pretty annoying.)