Matt, you're right, putting Visible Fields at the top does show all the Visible Fields. But that is simply a showing of the fields/columns of the view, same as the old Tag Windows, so I don't see a benefit, unless the resulting list of Visible Fields can be further edited (add/remove/move). I sort-of see a way, if Visible Fields is dragged to the top of the Fields, I can then remove or reorder fields below it. But if Visible Fields is not explicitly added, or it is at the bottom where it goes when first added, diddling with the other fields seems to do nothing. Also, it's not obvious that adding Visible Fields is necessary or useful, since then I first open Customize, it is already listing all the visible fields of the view...right?
To use the true Customize feature -- select individual desired fields in desired order to see and edit -- seems to result in a blank Tag Window. I can't find a way to see any individual Fields that are directly Added, but doing this seems to be the only reason to use the new Tag Window mode.
If my confusion makes any sense (oxymoron), can you explain the intended uses and outcome of the new mode, and steps to achieve it? I assume whatever is in the left-side Fields list is what I'll see in the Tag Window, controllable per-template, but it's not happening for me.