SwiftUI - How to make navigation bar title editable (without changing any other behavior)?

I really like the look of the navigation bar title in SwiftUI, and I like that it appears just below the safe area, but appears in the principal part of the toolbar when you scroll down. I'm wondering how to completely replicate this look and behavior but make it editable by the user (most likely through a textfield?)

I've tried

.toolbar {
  ToolbarItem(placement: .principal) {
    TextField("Navigation Title", text: $mainTitle)
  }
}

But this simply places the title in the toolbar at all times, rather than only when you scroll slightly.

Any ideas?