ghostty/macos/Sources/Features/Terminal/Window Styles/TitlebarTabsTahoeTerminalWindow.swift

import AppKit
import SwiftUI

/// `macos-titlebar-style = tabs` for macOS 26 (Tahoe) and later.
///
/// This inherits from transparent styling so that the titlebar matches the background color
/// of the window.
class TitlebarTabsTahoeTerminalWindow: TransparentTitlebarTerminalWindow, NSToolbarDelegate {
    /// The view model for SwiftUI views
    private var viewModel = ViewModel()

    deinit {
        tabBarObserver = nil
    }

    // MARK: NSWindow

    override var title: String {
        didSet {
            viewModel.title = title
        }
    }

    override func awakeFromNib() {
        super.awakeFromNib()

        // We must hide the title since we're going to be moving tabs into
        // the titlebar which have their own title.
        titleVisibility = .hidden

        // Create a toolbar
        let toolbar = NSToolbar(identifier: "TerminalToolbar")
        toolbar.delegate = self
        toolbar.centeredItemIdentifiers.insert(.title)
        self.toolbar = toolbar
        toolbarStyle = .unifiedCompact
    }

    override func becomeMain() {
        super.becomeMain()

        // Check if we have a tab bar and set it up if we have to. See the comment
        // on this function to learn why we need to check this here.
        setupTabBar()
    }

    // This is called by macOS for native tabbing in order to add the tab bar. We hook into
    // this, detect the tab bar being added, and override its behavior.
    override func addTitlebarAccessoryViewController(_ childViewController: NSTitlebarAccessoryViewController) {
        // If this is the tab bar then we need to set it up for the titlebar
        guard isTabBar(childViewController) else {
            super.addTitlebarAccessoryViewController(childViewController)
            return
        }

        // Some setup needs to happen BEFORE it is added, such as layout. If
        // we don't do this before the call below, we'll trigger an AppKit
        // assertion.
        childViewController.layoutAttribute = .right

        super.addTitlebarAccessoryViewController(childViewController)

        // Setup the tab bar to go into the titlebar.
        DispatchQueue.main.async {
            // HACK: wait a tick before doing anything, to avoid edge cases during startup... :/
            // If we don't do this then on launch windows with restored state with tabs will end
            // up with messed up tab bars that don't show all tabs.
            self.setupTabBar()
        }
    }

    override func removeTitlebarAccessoryViewController(at index: Int) {
        guard let childViewController = titlebarAccessoryViewControllers[safe: index],
                isTabBar(childViewController) else {
            super.removeTitlebarAccessoryViewController(at: index)
            return
        }

        super.removeTitlebarAccessoryViewController(at: index)

        removeTabBar()
    }

    // MARK: Tab Bar Setup

    private var tabBarObserver: NSObjectProtocol? {
        didSet {
            // When we change this we want to clear our old observer
            guard let oldValue else { return }
            NotificationCenter.default.removeObserver(oldValue)
        }
    }

    /// Take the NSTabBar that is on the window and convert it into titlebar tabs.
    ///
    /// Let me explain more background on what is happening here. When a tab bar is created, only the
    /// main window actually has an NSTabBar. When an NSWindow in the tab group gains main, AppKit
    /// creates/moves (unsure which) the NSTabBar for it and shows it. When it loses main, the tab bar
    /// is removed from the view hierarchy.
    ///
    /// We can't reliably detect this via `addTitlebarAccessoryViewController` because AppKit
    /// creates an accessory view controller for every window in the tab group, but only attaches
    /// the actual NSTabBar to the main window's accessory view.
    ///
    /// The best way I've found to detect this is to search for and setup the tab bar anytime the
    /// window gains focus. There are probably edge cases to check but to resolve all this I made
    /// this function which is idempotent to call.
    ///
    /// There are more scenarios to look out for and they're documented within the method.
    func setupTabBar() {
        // We only want to setup the observer once
        guard tabBarObserver == nil else { return }

        // Find our tab bar. If it doesn't exist we don't do anything.
        guard let tabBar = contentView?.rootView.firstDescendant(withClassName: "NSTabBar") else { return }

        // View model updates must happen on their own ticks.
        DispatchQueue.main.async {
            self.viewModel.hasTabBar = true
        }

        // Find our clip view
        guard let clipView = tabBar.firstSuperview(withClassName: "NSTitlebarAccessoryClipView") else { return }
        guard let accessoryView = clipView.subviews[safe: 0] else { return }
        guard let titlebarView = clipView.firstSuperview(withClassName: "NSTitlebarView") else { return }
        guard let toolbarView = titlebarView.firstDescendant(withClassName: "NSToolbarView") else { return }

        // The container is the view that we'll constrain our tab bar within.
        let container = toolbarView

        // The padding for the tab bar. If we're showing window buttons then
        // we need to offset the window buttons.
        let leftPadding: CGFloat = switch(self.derivedConfig.macosWindowButtons) {
        case .hidden: 0
        case .visible: 70
        }

        // Constrain the accessory clip view (the parent of the accessory view
        // usually that clips the children) to the container view.
        clipView.translatesAutoresizingMaskIntoConstraints = false
        accessoryView.translatesAutoresizingMaskIntoConstraints = false

        // Setup all our constraints
        NSLayoutConstraint.activate([
            clipView.leftAnchor.constraint(equalTo: container.leftAnchor, constant: leftPadding),
            clipView.rightAnchor.constraint(equalTo: container.rightAnchor),
            clipView.topAnchor.constraint(equalTo: container.topAnchor, constant: 2),
            clipView.heightAnchor.constraint(equalTo: container.heightAnchor),
            accessoryView.leftAnchor.constraint(equalTo: clipView.leftAnchor),
            accessoryView.rightAnchor.constraint(equalTo: clipView.rightAnchor),
            accessoryView.topAnchor.constraint(equalTo: clipView.topAnchor),
            accessoryView.heightAnchor.constraint(equalTo: clipView.heightAnchor),
        ])

        clipView.needsLayout = true
        accessoryView.needsLayout = true

        // Setup an observer for the NSTabBar frame. When system appearance changes or
        // other events occur, the tab bar can temporarily become zero-sized. When this
        // happens, we need to remove our custom constraints and re-apply them once the
        // tab bar has proper dimensions again to avoid constraint conflicts.
        tabBar.postsFrameChangedNotifications = true
        tabBarObserver = NotificationCenter.default.addObserver(
            forName: NSView.frameDidChangeNotification,
            object: tabBar,
            queue: .main
        ) { [weak self] _ in
            guard let self else { return }

            // Check if either width or height is zero
            guard tabBar.frame.size.width == 0 || tabBar.frame.size.height == 0 else { return }

            // Remove the observer so we can call setup again.
            self.tabBarObserver = nil

            // Wait a tick to let the new tab bars appear and then set them up.
            DispatchQueue.main.async {
                self.setupTabBar()
            }
        }
    }

    func removeTabBar() {
        // View model needs to be updated on another tick because it
        // triggers view updates.
        DispatchQueue.main.async {
            self.viewModel.hasTabBar = false
        }

        // Clear our observations
        self.tabBarObserver = nil
    }

    // MARK: NSToolbarDelegate

    func toolbarAllowedItemIdentifiers(_ toolbar: NSToolbar) -> [NSToolbarItem.Identifier] {
        return [.title, .flexibleSpace, .space]
    }

    func toolbarDefaultItemIdentifiers(_ toolbar: NSToolbar) -> [NSToolbarItem.Identifier] {
        return [.flexibleSpace, .title, .flexibleSpace]
    }

    func toolbar(_ toolbar: NSToolbar,
                 itemForItemIdentifier itemIdentifier: NSToolbarItem.Identifier,
                 willBeInsertedIntoToolbar flag: Bool) -> NSToolbarItem? {
        switch itemIdentifier {
        case .title:
            let item = NSToolbarItem(itemIdentifier: .title)
            item.view = NSHostingView(rootView: TitleItem(viewModel: viewModel))
            item.visibilityPriority = .user
            item.isEnabled = true

            // This is the documented way to avoid the glass view on an item.
            // We don't want glass on our title.
            item.isBordered = false
            
            return item
        default:
            return NSToolbarItem(itemIdentifier: itemIdentifier)
        }
    }

    // MARK: SwiftUI

    class ViewModel: ObservableObject {
        @Published var title: String = "👻 Ghostty"
        @Published var hasTabBar: Bool = false
    }
}

extension NSToolbarItem.Identifier {
    /// Displays the title of the window
    static let title = NSToolbarItem.Identifier("Title")
}

extension TitlebarTabsTahoeTerminalWindow {
    /// Displays the window title
    struct TitleItem: View {
        @ObservedObject var viewModel: ViewModel

        var title: String {
            // An empty title makes this view zero-sized and NSToolbar on macOS
            // tahoe just deletes the item when that happens. So we use a space
            // instead to ensure there's always some size.
            return viewModel.title.isEmpty ? " " : viewModel.title
        }

        var body: some View {
            if !viewModel.hasTabBar {
                Text(title)
                    .lineLimit(1)
                    .truncationMode(.tail)
            } else {
                // 1x1.gif strikes again! For real: if we render a zero-sized
                // view here then the toolbar just disappears our view. I don't
                // know.
                Color.clear.frame(width: 1, height: 1)
            }
        }
    }
}