Bitbucket is much better IME. It has a better handle on the current state of the review rather than offering you the whole diff to re-review whenever someone rebases, and it loses comments much less than github (indeed not at all, in my experience).

What's wrong with reviewees marking comments resolved? That allows anyone reading to see what issues are "open" or already addressed at a glance.

Code review isn't adversarial, so it's not like the reviewee is closing comments to shove bad code through and they need approvals anyway.

Not bad if you have a junior that knows they are a junior, bad otherwise.

My pet peeve with bitbucket was that when doing code review, you couldn't send all comments and overall review in a batch, every single comment was received in real time by the author. Sometimes I would be still reviewing code when the author pushed fixes to comments I've made before and that still might need other changes. After 15 years they improved that 2 weeks ago. I'm glad it's better, but I hate it with a burning passion.

It's when they resolve it without verifying if their solution was good