Fade traffic lights with controls in macOS player window

In the separate player window the traffic-light buttons stayed visible
while the controls overlay was hidden. Animate the standard window
buttons' alpha in sync with controls visibility, QuickTime-style.

Applies only to the dedicated player window (inline sheet and overlay
fullscreen presentations keep their parent window's chrome). Buttons
are handed back at full alpha when entering native fullscreen, when the
controls unmount (PiP, debug overlay), and when the view re-parents to
another window.

Yattee/Views/Player/macOS/MacOSPlayerControlsView.swift

@@ -300,6 +300,27 @@ struct MacOSPlayerControlsView: View {
                 showControls = nil
             }
         }
+        .onChange(of: shouldShowControls) { _, visible in
+            setTrafficLightsVisible(visible)
+        }
+        .onChange(of: hostWindow) { oldWindow, _ in
+            // Restore the old window's chrome when re-parented (sheet <->
+            // floating window), then sync the new window to the current state.
+            if let oldWindow { applyTrafficLightAlpha(1, to: oldWindow, animated: false) }
+            setTrafficLightsVisible(shouldShowControls, animated: false)
+        }
+        .onReceive(NotificationCenter.default.publisher(for: NSWindow.willEnterFullScreenNotification)) { note in
+            // In fullscreen the system owns titlebar visibility (hover at the
+            // top edge reveals it) — hand the buttons back at full alpha.
+            if let window = note.object as? NSWindow, window === hostWindow {
+                applyTrafficLightAlpha(1, to: window, animated: false)
+            }
+        }
+        .onReceive(NotificationCenter.default.publisher(for: NSWindow.didExitFullScreenNotification)) { note in
+            if let window = note.object as? NSWindow, window === hostWindow {
+                setTrafficLightsVisible(shouldShowControls, animated: false)
+            }
+        }
         .onChange(of: playerState.playbackState) { oldState, newState in
             // When playback starts, start hide timer
             if newState == .playing && shouldShowControls {
@@ -326,6 +347,9 @@ struct MacOSPlayerControlsView: View {
         }
         .onDisappear {
             removeKeyboardMonitor()
+            // The controls unmount while the window stays up (PiP, debug
+            // overlay) — don't leave the window without its close button.
+            if let hostWindow { applyTrafficLightAlpha(1, to: hostWindow, animated: false) }
         }
         .task {
             await loadLayout()
@@ -413,6 +437,34 @@ struct MacOSPlayerControlsView: View {
             }
     }
 
+    // MARK: - Traffic Lights
+
+    /// Fades the hosting window's traffic-light buttons together with the
+    /// controls overlay, QuickTime-style. Only applies to the dedicated
+    /// separate player window — the inline sheet's parent window keeps its own
+    /// chrome — and never in fullscreen, where the system owns the titlebar.
+    private func setTrafficLightsVisible(_ visible: Bool, animated: Bool = true) {
+        guard let window = hostWindow,
+              window === ExpandedPlayerWindowManager.shared.currentPlayerWindow,
+              !window.styleMask.contains(.fullScreen) else { return }
+        applyTrafficLightAlpha(visible ? 1 : 0, to: window, animated: animated)
+    }
+
+    private func applyTrafficLightAlpha(_ alpha: CGFloat, to window: NSWindow, animated: Bool) {
+        let buttons: [NSWindow.ButtonType] = [.closeButton, .miniaturizeButton, .zoomButton]
+        let views = buttons.compactMap { window.standardWindowButton($0) }
+        guard !views.isEmpty else { return }
+        if animated {
+            NSAnimationContext.runAnimationGroup { context in
+                context.duration = 0.2
+                context.timingFunction = CAMediaTimingFunction(name: .easeInEaseOut)
+                views.forEach { $0.animator().alphaValue = alpha }
+            }
+        } else {
+            views.forEach { $0.alphaValue = alpha }
+        }
+    }
+
     // MARK: - Layout Loading
 
     private func loadLayout() async {