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NavigationView ignores preference keys from its root view when popping back to the root view.

Additionally, the NavigationView returns the default value for a preference key before returning the root view's preference key value on the NavigationView's first appearance.

Given:

* The code below
* The user input: push, push, pop, pop, push, push, pop, pop

Expected output: 

0, 1, 2, 1, 0, 1, 2, 1, 0

Output on iOS 15.4 (device and simulator): 

-1, 0, 1, 2, 1, 2, 1

Code:

```swift
import SwiftUI

@main
struct navigation_view_preference_keyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

struct ContentView: View {
    var body: some View {
        NavigationView {
            LeafView(count: 0)
        }
        .onPreferenceChange(CountKey.self) { newValue in
            print(newValue)
        }
    }
}

struct LeafView: View {
    let count: Int

    var body: some View {
        NavigationLink("Push") {
            LeafView(count: count + 1)
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .navigationTitle("\(count)")
        .preference(key: CountKey.self, value: count)
    }
}

struct CountKey: PreferenceKey {
    static var defaultValue: Int = -1

    static func reduce(value: inout Int, nextValue: () -> Int) {
        value = nextValue()
    }
}
```

SEO: NavigationView NavigationLink PreferenceKey onPreferenceChange preferenceKey preference(key:value:)