obviously all collections are part of "My Library", so you can select any item, but you cannot restrict the search to a collection, no. (You can switch to the old view by clicking on the little triangle on the left of the quick format bar, though).

I do think this is an important feature of the classic dialog. Not sure the best way to offer it in the new one. The goal, I would say, should be to get rid of the classic dialog altogether.

I guess the main question is whether it's important to be able to actually browse items in collections or just to restrict the search to a given collection.

people in workshops also tell me that browsing is important, many of them to such a degree that they include switching to the classic view as a recommendation in their workshops/libguides (against my protestations, I shall add, but this gives you a sense of how important they think it is).

Right. So I guess one option would be to integrate the tree views from the classic dialog into the quick format dialog such that you click an icon or button (or select from the current menu, but this probably needs to be more discoverable) and they appear in the same place as the current search results panel. The search box would then function similarly to the quick search bar in the classic view, scoped to the current collection and filtering the items list, but Return would retain its current magic (and would be equivalent to double-clicking an item in the list).

Alternatively, there could just be a collections drop-down somewhere (to the right of the search bar?) to scope the search, but pressing Return with no input would display the search results panel with all items in the selected library/collection/search, which you could then scroll through.