fix: keep macOS traffic lights within the navigation rail

ui/flutter/macos/Runner/MainFlutterWindow.swift

@@ -4,6 +4,10 @@ import desktop_multi_window
 import window_manager
 
 class MainFlutterWindow: NSWindow {
+  // Keep this in sync with AppDesignTokens.railWidth in Flutter.
+  private static let navigationRailWidth: CGFloat = 68
+  private static let trafficLightGap: CGFloat = 6
+
   override func awakeFromNib() {
     let flutterViewController = FlutterViewController()
     let windowFrame = self.frame
@@ -16,10 +20,53 @@ class MainFlutterWindow: NSWindow {
     }
 
     super.awakeFromNib()
+
+    for name in [NSWindow.didResizeNotification, NSWindow.didBecomeKeyNotification,
+                 NSWindow.didExitFullScreenNotification,
+                 NSWindow.didChangeBackingPropertiesNotification] {
+      NotificationCenter.default.addObserver(
+        self, selector: #selector(refreshTrafficLightLayout(_:)), name: name, object: self)
+    }
+    layoutIfNeeded()
+  }
+
+  override func layoutIfNeeded() {
+    super.layoutIfNeeded()
+    // Let AppKit position controls in its own full-screen title bar.
+    guard !styleMask.contains(.fullScreen) else { return }
+
+    let buttons = [NSWindow.ButtonType.closeButton, .miniaturizeButton, .zoomButton]
+      .compactMap { standardWindowButton($0) }
+    guard buttons.count == 3 else { return }
+
+    let width = buttons.reduce(CGFloat.zero) { $0 + $1.frame.width }
+      + Self.trafficLightGap * CGFloat(buttons.count - 1)
+    var x = (Self.navigationRailWidth - width) / 2
+    guard x >= 0 else { return }
+
+    // Keep native sizes, vertical positions and button behavior intact.
+    for button in buttons {
+      var origin = button.frame.origin
+      origin.x = x
+      if button.frame.origin != origin {
+        button.setFrameOrigin(origin)
+      }
+      x += button.frame.width + Self.trafficLightGap
+    }
+  }
+
+  @objc private func refreshTrafficLightLayout(_ notification: Notification) {
+    // AppKit can reset button frames while finishing a resize or style change.
+    DispatchQueue.main.async { [weak self] in self?.layoutIfNeeded() }
+  }
+
+  deinit {
+    NotificationCenter.default.removeObserver(self)
   }
 
   override public func order(_ place: NSWindow.OrderingMode, relativeTo otherWin: Int) {
     super.order(place, relativeTo: otherWin)
     hiddenWindowAtLaunch()
+    layoutIfNeeded()
   }
 }