fix: align titlebar icons with traffic-light buttons (#1754)

The titlebar control icons (sidebar, notifications, new tab) were
vertically misaligned — centered within the full titlebar+tab-bar
area instead of the titlebar alone.

Use the close button's superview height as the true titlebar height
so the icons sit at the same vertical center as the traffic lights,
matching the behavior of apps like Slack.

Sources/Update/UpdateTitlebarAccessory.swift

@@ -881,9 +881,22 @@ final class TitlebarControlsAccessoryViewController: NSTitlebarAccessoryViewCont
         contentSize = cachedFittingSize ?? .zero
 
         guard contentSize.width > 0, contentSize.height > 0 else { return }
-        let titlebarHeight = view.window.map { window in
-            window.frame.height - window.contentLayoutRect.height
-        } ?? contentSize.height
+        // Use the traffic-light close button's superview height as the true
+        // titlebar height. This excludes the tab bar so the icons align with
+        // the traffic-light buttons (like Slack does) instead of centering in
+        // the full non-content area which includes the tab strip.
+        let titlebarHeight: CGFloat = {
+            if let window = view.window,
+               let closeButton = window.standardWindowButton(.closeButton),
+               let titlebarView = closeButton.superview,
+               titlebarView.frame.height > 0 {
+                return titlebarView.frame.height
+            }
+            // Fallback: derive from the window geometry.
+            return view.window.map { window in
+                window.frame.height - window.contentLayoutRect.height
+            } ?? contentSize.height
+        }()
         let containerHeight = max(contentSize.height, titlebarHeight)
         let yOffset = max(0, (containerHeight - contentSize.height) / 2.0)
         let nextLayoutSnapshot = TitlebarControlsLayoutSnapshot(