When the sidebar is showing the Binder, Ctrl+Tab will rotate the focus between the sidebar and one or both of the editors, left to right.
But when the sidebar is showing a Collection, Ctrl-Tab does nothing at all once the focus moves to the single editor or the right-side split.
Is there a reason for that?
As different as the full Binder is from collections, much of the work you do with one you also do with the other. Navigation should work consistently between the sidebar and the editors, regardless of what happens to be on top in the sidebar.
Not unrelated, the terminology here is ambiguous if not messy. The thing sometimes called the sidebar is also called the Binder, but Binder is actually just one particular view (the “master collection”) within the sidebar, along with regular Collections and search results. Calling the sidebar Binder seems like a legacy convention from earlier versions of Scrivener.
In the View–>Collections menu, Binder is the first of the items listed along with search results and all user-created collections (again, Binder = master collection.).
But when a Collection is displayed in the sidebar, going to the View–>Move Focus To menu and clicking Binder moves the focus to the sidebar and leaves collection in focus.
The View–>Layout menu has a show/hide Binder item with the shortcut Ctrl+Shift+B, but what it really does is show/hide the sidebar, and (like the Move Focus To Binder) will leave any focused collection unchanged.
Obviously, none of these things (and there are lots more) keep Scriv from being useable, powerful, and valuable. It really is a great program. But I have to wonder what the connection is between inconsistent terminology and labeling on the one hand, and inconsistent operation, such as the above Ctrl-Tab issue, on the other. (Maybe there’s none at all, and there’s a solid technical reason why Ctrl-Tab just can’t move from an editor back to the sidebar when the sidebar is showing a collection. But that wouldn’t make the other inconsistencies any less inconsistent.)