GitHub introduced a new feature : Mark files as viewed

Try as you might, you can’t always avoid huge pull requests. When you search through or need to review one, it can take hours—and before you know it you’ve forgotten where you left off. Now, there’s an easy way to keep track of your progress by marking files as viewed.

How it works
You can quickly check the progress of your review from the top of the pull request. This is where you’ll see how many files you’ve viewed and how many remain. You can mark each file as viewed, and they’ll automatically collapse, making code review quicker.

If a file is changed after you’ve viewed it, the status is removed and a “Changed since last view” badge appears, so you never miss a new change.

Learn more about marking files as viewed