While it's a bit tucked away, when you are viewing the changes for a PR, there is a dropdown in the top left for “Changes from <all commits>”. You can select an

While it's a bit tucked away, when you are viewing the changes for a PR, there is a dropdown in the top left for “Changes from <all commits>”. You can select any range there to view changes from that range of commits. When you've left a review, you also generally get a “View Changes” button in the conversation that takes you to the changes since you last viewed, if new changes are pushed. The issue with this particular link is it tends to be ephemeral.