How can I know which dwellers are idle without scrolling through my list of dwellers? I'm talking about those who don't have an assigned job and are just walking.
Without using the list, you can spot them more easily by zooming out (to the point where you get 2D graphics and no working animations). They will likely be the only ones moving at all.
Otherwise, remember that the dweller list can be sorted by tapping the "Job" column header. This way, everyone is grouped by their jobs, and it's easy to find the "Coffee Break" people.