A TabView-like implementation for getting a "tabs in the toolbar" style of tab bar on macOS.

ToolbarTabView.swift

import SwiftUI

struct ToolbarTabView<Content: View>: View {
    let content: () -> Content
    
    init(@ViewBuilder content: @escaping () -> Content) {
        self.content = content
    }
    
    var body: some View {
        _VariadicView.Tree(_ToolbarTabView_Layout(), content: content)
    }
}

private struct _ToolbarTabView: NSViewControllerRepresentable {
    let children: _VariadicView.Children
    
    func makeNSViewController(context: Context) -> NSTabViewController {
        let viewController = NSTabViewController()
        viewController.tabStyle = .toolbar
        return viewController
    }
    
    func updateNSViewController(_ nsViewController: NSTabViewController, context: Context) {
        context.coordinator.updateChildren(children, inViewController: nsViewController)
    }
    
    func makeCoordinator() -> Coordinator {
        return Coordinator()
    }
    
    final class Coordinator {
        var children: [AnyHashable: NSTabViewItem] = [:]
        
        func updateChildren(_ children: _VariadicView.Children, inViewController viewController: NSTabViewController) {
            for (key, tabViewItem) in self.children {
                if children.contains(where: { $0.id == key }) { continue }
                
                self.children.removeValue(forKey: key)
                viewController.removeTabViewItem(tabViewItem)
            }
            
            for (index, child) in zip(children.indices, children) {
                let toolbarTabItem = child[ToolbarTabItem.self]
            
                if let tabViewItem = self.children[child.id] {
                    viewController.removeTabViewItem(tabViewItem)
                    
                    updateTabViewItem(tabViewItem, with: toolbarTabItem)
                    
                    viewController.insertTabViewItem(tabViewItem, at: index)
                } else {
                    let tabViewItem = makeTabViewItem(for: child)
                    
                    updateTabViewItem(tabViewItem, with: toolbarTabItem)
                    
                    viewController.insertTabViewItem(tabViewItem, at: index)
                }
            }
        }
        
        private func updateTabViewItem(_ tabViewItem: NSTabViewItem, with toolbarTabItem: ToolbarTabItem?) {
            tabViewItem.label = toolbarTabItem?.title ?? UUID().uuidString
            tabViewItem.image = toolbarTabItem?.image
        }
        
        private func makeTabViewItem(for child: _VariadicView.Children.Element) -> NSTabViewItem {
            let tabViewItem = NSTabViewItem()
            tabViewItem.viewController = NSHostingController(rootView: child)
            
            self.children[child.id] = tabViewItem
            
            return tabViewItem
        }
    }
}

private struct _ToolbarTabView_Layout: _VariadicView_ViewRoot {
    func body(children: _VariadicView.Children) -> some View {
        _ToolbarTabView(children: children)
    }
}

// MARK: -

private struct ToolbarTabItem: Equatable {
    let title: String
    let image: NSImage
}

extension ToolbarTabItem: _ViewTraitKey {
    static var defaultValue: ToolbarTabItem?
}

extension View {
    func toolbarTabItem(title: String, image: NSImage) -> some View {
        _trait(ToolbarTabItem.self, .init(title: title, image: image))
    }
    
    func toolbarTabItem(title: String, systemSymbolName: String) -> some View {
        self.toolbarTabItem(title: title, image: NSImage(systemSymbolName: systemSymbolName, accessibilityDescription: nil)!)
    }
}