One of my students updated Animate and when he came online for the session he had Thumbnails (similar to any editing software) on the timeline instead of the age old ·
I asked him how he did it, and he did not know, said it was an accident that he liked, but couldn't remember what he did "wrong."
I can't seem to find that option in the menus, or the icons anywhere around the timeline.
I think he has selected either 'Preview' or 'Preview in Context' when you click on the burger icon highlighted below and it opens up the menu with options I just mentioned...
Hope this is what you are looking for? I never use it so can't be sure.