Preview now runs a lesson the way a learner would experience it. Required interactions and quizzes hold you on the page as they would a learner, so what you see in preview is what your audience gets, rather than a version with the rules relaxed.
Where that would slow down a review, controls marked Author only let you skip a requirement or go straight to the completion screen. Learners never see them.
Restarting works in preview, the completion screen and certificate can be previewed, and course preview tracks progress across its lessons, so you can check a whole course from first page to certificate before sharing it.