Navigation wastes vertical space on iPhone

We've had some struggles getting our master/detail view to work well on both iPhone and iPad. The current iteration is mostly OK. It looks good on iPad / Mac, and it works ok enough on iPhone. But it does waste valuable space on iPhone by allocating two "header" areas at the top of the screen, because we have two different navigation stacks going on: one for the master/detail view, and one inside the "detail" column.

SwiftUI provides NavigationSplitView - and in fact they have deprecated the older usage of NavigationView. Unfortunately the last time we tried NavigationSplitView with Xcode 14, it didn't work at all.

Now that we have Xcode 15, we should at least try using the new view. If it works, then great, our problem should be solved. If it still doesn't work, then we might need to hack something together ourselves with GeometryReader.