简单！使用if语句

struct ContentView: View {
    @State private var use2Column: bool = false
    @State private var columnVisibility: NavigationSplitViewVisibility = .all
    var body: some View {
        Group {
        if use2Column {
            NavigationSplitView(columnVisibility: $columnVisibility) {
                // sidebar
            } detail: {
                // detail
            }
        } else {
            NavigationSplitView(columnVisibility: $columnVisibility) {
                // sidebar
            } content: {
                // content
            } detail: {
                // detail
            }
        }
    }
    .navigationSplitViewStyle(.balanced)
}

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}
虽然根据显示的数据大小，可能会/可能不会平滑过渡