In Tools>Preferences>Details view there are two lists - "Available" and "Chosen". The height of these lists allow for a maximum of eight items. Because (currently) there are twelve items actually available, this results in the need to use a scroll bar to see the extra items.
There appears to be plenty of room to make the height much larger so that the need for a scroll bar disappears.
And some users might not immediately understand that the way to move an item from "Available" to "Chosen" or vice versa is just to "drag&drop". There would be room to add a short hint
One thing I miss from the old "Explorer" view is the ability to see more thumbnails at a time in small rows and columns and be able to rearrange the order with "Drag&Drop".
I guess, though, it might be too difficult to have that as an alternative view.
How do you mean? Can't you just use the zoom slider to make the thumbnails smaller?
Apart from switching "Label type" / view mode to "Filename", also reduce the thumbnail image size using the zoom slider. This will get you much more overview. Also, in order to not having to drag and drop objects over large distances, involving a lot of scrolling, try this approach instead:
1) Select the objects you wish to move
2) Hit CTRL+X (CMD+X on Mac) to mark them for "Cut"
3) Scroll to the destination location.
4) Position the mouse between two thumbnails, where you wish the cut-out objects to appear.
5) Hit CTRL+V (CMD+V)