NavigationStack layout shift

The SkipUI NavigationStack has a janky layout shift when it first appears. This repros in the Skip Lite Showcase.

In the video below, watch it at 0.5 speed. (In Chrome, click the "..." menu to select Playback Speed). When selecting the Showcase tab, the Search bar appears and then scoots up a bit.

https://github.com/user-attachments/assets/721ac011-26ad-4d82-ae60-c11f7f638e09

You can also repro this in a trivial Skip app. I ran skip init --git-repo --appid=bundle.id nav-stack-layout-shift-repro NavStackLayoutShiftRepro and then replaced ContentView.swift with this:

import SwiftUI

struct ContentView: View {

    var body: some View {
        NavigationStack {
            VStack {
                Text("Hello, World!")
                Spacer()
            }
            .navigationTitle("Hello")
        }
    }
}

This simple app has two layout shifts. "Hello World" first appears at the very top of the screen, then scoots down to below where the nav bar would be, then scoots up again as the nav bar resizes smaller.

https://github.com/user-attachments/assets/071f1303-d37f-410d-8357-a665d2735c08

Start by reproducing the issue with the provided ContentView.swift in a trivial Skip app, then compare it with the Skip Lite Showcase NavigationStack. Trace the initial placement and resizing of the navigation bar and content; done means the first appearance has no visible layout shifts in both reproductions.