osx supports prefering system chrome buttons and also can turn chrome buttons on or off.

native/Avalonia.Native/inc/avalonia-native.h

@@ -209,8 +209,9 @@ enum AvnExtendClientAreaChromeHints
     AvnChromeHintsNoChrome,
     AvnChromeHintsSystemTitleBar = 0x01,
     AvnChromeHintsManagedChromeButtons = 0x02,
-    AvnChromeHintsPreferSystemChromeButtons = 0x04,
+    AvnChromeHintsSystemChromeButtons = 0x04,
     AvnChromeHintsOSXThickTitleBar = 0x08,
+    AvnChromeHintsDefault = AvnChromeHintsSystemTitleBar | AvnChromeHintsSystemChromeButtons,
 };
 
 AVNCOM(IAvaloniaNativeFactory, 01) : IUnknown

native/Avalonia.Native/src/OSX/window.mm

@@ -488,7 +488,7 @@ private:
     WindowImpl(IAvnWindowEvents* events, IAvnGlContext* gl) : WindowBaseImpl(events, gl)
     {
         _isClientAreaExtended = false;
-        _extendClientHints = AvnChromeHintsSystemTitleBar;
+        _extendClientHints = AvnChromeHintsDefault;
         _fullScreenActive = false;
         _canResize = true;
         _decorations = SystemDecorationsFull;
@@ -507,8 +507,17 @@ private:
             if ([subview isKindOfClass:NSClassFromString(@"NSTitlebarContainerView")]) {
                 NSView *titlebarView = [subview subviews][0];
                 for (id button in titlebarView.subviews) {
-                    if ([button isKindOfClass:[NSButton class]]) {
-                        [button setHidden: (_decorations != SystemDecorationsFull)];
+                    if ([button isKindOfClass:[NSButton class]])
+                    {
+                        if(_isClientAreaExtended)
+                        {
+                            [button setHidden: !(_extendClientHints & AvnChromeHintsSystemChromeButtons)];
+                        }
+                        else
+                        {
+                            [button setHidden: (_decorations != SystemDecorationsFull)];
+                        }
+                        
                         [button setWantsLayer:true];
                     }
                 }
@@ -806,6 +815,8 @@ private:
         
         [Window setIsExtended:enable];
         
+        HideOrShowTrafficLights();
+        
         UpdateStyle();
         
         return S_OK;

samples/ControlCatalog/ViewModels/MainWindowViewModel.cs

@@ -70,6 +70,11 @@ namespace ControlCatalog.ViewModels
                 {
                     ChromeHints = ExtendClientAreaChromeHints.NoChrome | ExtendClientAreaChromeHints.OSXThickTitleBar;
 
+                    if(x.Item1)
+                    {
+                        ChromeHints |= ExtendClientAreaChromeHints.SystemChromeButtons;
+                    }
+
                     if(x.Item2)
                     {
                         ChromeHints |= ExtendClientAreaChromeHints.ManagedChromeButtons;

src/Avalonia.Controls/Platform/ExtendClientAreaChromeHints.cs

@@ -9,9 +9,11 @@ namespace Avalonia.Platform
         Default = SystemTitleBar,        
         SystemTitleBar = 0x01,
         ManagedChromeButtons = 0x02,
-        PreferSystemChromeButtons = 0x04,
+        SystemChromeButtons = 0x04,
 
         OSXThickTitleBar = 0x08,
+
+        PreferSystemChromeButtons = 0x10,
         
         AdaptiveChromeWithTitleBar = SystemTitleBar | PreferSystemChromeButtons,
         AdaptiveChromeWithoutTitleBar = PreferSystemChromeButtons,

src/Avalonia.Native/WindowImpl.cs

@@ -149,6 +149,11 @@ namespace Avalonia.Native
 
         public void SetExtendClientAreaChromeHints(ExtendClientAreaChromeHints hints)
         {
+            if(hints.HasFlag(ExtendClientAreaChromeHints.PreferSystemChromeButtons))
+            {
+                hints |= ExtendClientAreaChromeHints.SystemChromeButtons;
+            }
+            
             _native.SetExtendClientAreaHints ((AvnExtendClientAreaChromeHints)hints);
         }