I find the "View Changes" feature on GitHub PRs really useful - the button that essentially says "open the 'Files Changed' tab, comparing the current HEAD
with the last commit I reviewed". This allows you to comment on exact changes that were made between now and the previous review. This button disappears after clicking it though, and (AFAIK) there's no easy way to access this functionality after the button disappears.
The URL format from clicking this button can be created manually, so you can choose to compare two different commits in the context of a PR review. The URL looks something like this:
https://github.com/mattermost/mattermost-plugin-github/pull/779/files/14c9e8b..0dfcb28
https://github.com/mattermost/mattermost-plugin-github/pull/779/files/14c9e8b..HEAD