When reviewing a PR, you should be able to author multiple comments, come back and tweak them, etc before publishing them to everyone else.

Github has this feature, and it makes a huge difference in my ability to provide coherent PR feedback, and to do so in a way that’s respectful of others’ time and feelings. (E.g. I may answer my own questions before getting to the end of the review, or may discover that one of my own comments was not a good one based on further reading and consideration of the PR). This feature facilitates efficient and constructive reviews.