The last reviewed sha is generally available on the PR page (not the changes page) when you force push. There should be a changes since review link somewhere near the push.

When reviewing, you can also mark individual files as reviewed (useful in larger reviews where you're incrementally reviewing files). If you do this, only files that are changed will be expanded when you come back to the review.

yeah, this is where my complaint is - github shows a "compare" button when I force push, but it's not linked to the PR review. The "file changed" status is often not granular enough - if I change one line, force push, the entire file gets marked as unreviewed. the github "changes since your last review" is commit-based, not sha-based.

what I want is something like graphite/gerritt/google's critique where after each force push, the review page shows only the true delta between the two shas (similar to the "compare" button in github, bu as a reviewable unit).

poked around on github, doesn't look like the stacked PR feature has affected this "changes since your last review" selector yet :(