You can still review changes per file in context in the new editor mode. The undo/keep buttons and next file buttons are just in the breadcrumbs area instead of overlay. It is not my preference, so I stay on the old editor, but the functionality in the new editor is not lost. I do wish they would maintain the old editor because I do prefer it.
Old editor (overlay):
New Editor (breadcrumb):
As you can see, they both have UI for navigating to the next file.
If you are not on a file that is part of the changes, then both show a button to “Review next file”
The breadcrumb buttons in the new editor are technically better, since they are not center aligned, so the undo and keep buttons don’t move around causing you to accidentally click the wrong one.