feat: Add API for fetching window Z-order

native/Avalonia.Native/src/OSX/WindowImpl.h

@@ -82,6 +82,8 @@ BEGIN_INTERFACE_MAP()
     virtual HRESULT GetExtendTitleBarHeight (double*ret) override;
 
     virtual HRESULT SetExtendTitleBarHeight (double value) override;
+    
+    virtual HRESULT GetWindowZOrder (long* zOrder) override;
 
     void EnterFullScreenMode ();
 

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

@@ -366,6 +366,18 @@ HRESULT WindowImpl::GetWindowState(AvnWindowState *ret) {
     }
 }
 
+HRESULT WindowImpl::GetWindowZOrder(long* zOrder) {
+    START_COM_CALL;
+    @autoreleasepool {
+        if (zOrder == nullptr) {
+            return E_POINTER;
+        }
+
+        *zOrder = [Window orderedIndex];
+        return S_OK;
+    }
+}
+
 HRESULT WindowImpl::TakeFocusFromChildren() {
     START_COM_CALL;
 

src/Avalonia.Controls/Platform/IWindowImpl.cs

@@ -98,6 +98,12 @@ namespace Avalonia.Platform
         /// </summary>
         Thickness OffScreenMargin { get; }
 
+        /// <summary>
+        /// Gets the window's z-order position. The highest value is the front, lower numbers are further back.
+        /// If the z-order can't be determined or the platform doesn't support window stacking, null is returned.
+        /// </summary>
+        IntPtr? ZOrder { get; }
+
         /// <summary>
         /// Starts moving a window with left button being held. Should be called from left mouse button press event handler.
         /// </summary>

src/Avalonia.Controls/Window.cs

@@ -428,6 +428,12 @@ namespace Avalonia.Controls
             set => PlatformImpl?.Move(value);
         }
 
+        /// <summary>
+        /// Gets the window's z-order position. The highest value is the front, lower numbers are further back.
+        /// If the z-order can't be determined or the platform doesn't support window stacking, null is returned.
+        /// </summary>
+        public IntPtr? WindowZOrder => PlatformImpl?.ZOrder;
+
         /// <summary>
         /// Starts moving a window with left button being held. Should be called from left mouse button press event handler
         /// </summary>

src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs

@@ -39,6 +39,7 @@ namespace Avalonia.DesignerSupport.Remote
 
         public double DesktopScaling => 1.0;
         public PixelPoint Position { get; set; }
+        public IntPtr? ZOrder => null;
         public Action<PixelPoint> PositionChanged { get; set; }
         public Action Deactivated { get; set; }
         public Action Activated { get; set; }

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -41,6 +41,7 @@ namespace Avalonia.DesignerSupport.Remote
         public IPopupImpl CreatePopup() => new WindowStub(this);
 
         public PixelPoint Position { get; set; }
+        public IntPtr? ZOrder => null;
         public Action<PixelPoint> PositionChanged { get; set; }
         public WindowState WindowState { get; set; }
         public Action<WindowState> WindowStateChanged { get; set; }

src/Avalonia.Native/WindowImpl.cs

@@ -103,6 +103,15 @@ namespace Avalonia.Native
 
         public Thickness OffScreenMargin { get; } = new Thickness();
 
+        public IntPtr? ZOrder
+        {
+            get
+            {
+                // macOS returns z-order in reverse order - the topmost window has the lowest z-order
+                return new IntPtr(-_native.WindowZOrder.ToInt64());
+            }
+        }
+
         private bool _isExtended;
         public bool IsClientAreaExtendedToDecorations => _isExtended;
 

src/Avalonia.Native/avn.idl

@@ -2,6 +2,7 @@
 @clr-access internal
 @clr-map bool int
 @clr-map u_int64_t ulong
+@clr-map long IntPtr
 @cpp-preamble @@
 #pragma once
 #include "com.h"
@@ -750,6 +751,7 @@ interface IAvnWindow : IAvnWindowBase
      HRESULT SetExtendClientAreaHints(AvnExtendClientAreaChromeHints hints);
      HRESULT GetExtendTitleBarHeight(double*ret);
      HRESULT SetExtendTitleBarHeight(double value);
+     HRESULT GetWindowZOrder(long*ret);
 }
 
 [uuid(939b6599-40a8-4710-a4c8-5d72d8f174fb)]

src/Avalonia.X11/X11Window.cs

@@ -1133,6 +1133,46 @@ namespace Avalonia.X11
             }
         }
 
+        public IntPtr? ZOrder
+        {
+            get
+            {
+                var stack = new Stack<IntPtr>();
+                var zOrder = 0;
+                stack.Push(_x11.RootWindow);
+
+                while (stack.Count > 0)
+                {
+                    var currentWindow = stack.Pop();
+
+                    if (currentWindow == _handle)
+                    {
+                        return new IntPtr(zOrder);
+                    }
+
+                    zOrder++;
+
+                    if (XQueryTree(_x11.Display, currentWindow, out _, out _,
+                            out var childrenPtr, out var childrenCount) != 0)
+                    {
+                        if (childrenPtr == IntPtr.Zero)
+                            continue;
+
+                        var children = (IntPtr*)childrenPtr;
+
+                        // Children are returned bottom to top, so we need to push them in reverse order
+                        // In order to traverse bottom children first
+                        for (int i = childrenCount - 1; i >= 0; i--)
+                            stack.Push(children[i]);
+
+                        XFree(childrenPtr);
+                    }
+                }
+
+                return null;
+            }
+        }
+
         public IMouseDevice MouseDevice => _mouse;
         public TouchDevice TouchDevice => _touch;
 

src/Headless/Avalonia.Headless/HeadlessWindowImpl.cs

@@ -100,6 +100,7 @@ namespace Avalonia.Headless
 
         public PixelPoint Position { get; set; }
         public Action<PixelPoint>? PositionChanged { get; set; }
+        public IntPtr? ZOrder => null;
         public void Activate()
         {
             Dispatcher.UIThread.Post(() => Activated?.Invoke(), DispatcherPriority.Input);

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -1274,6 +1274,20 @@ namespace Avalonia.Win32.Interop
         [DllImport("user32.dll")]
         public static extern int GetSystemMetrics(SystemMetric smIndex);
 
+        [DllImport("user32.dll", SetLastError = true)]
+        public static extern IntPtr GetWindow(IntPtr hWnd, GetWindowCommand uCmd);
+
+        public enum GetWindowCommand : uint
+        {
+            GW_CHILD = 5,
+            GW_ENABLEDPOPUP = 6,
+            GW_HWNDFIRST = 0,
+            GW_HWNDLAST = 1,
+            GW_HWNDNEXT = 2,
+            GW_HWNDPREV = 3,
+            GW_OWNER = 4
+        }
+
         [DllImport("user32.dll", SetLastError = true, EntryPoint = "GetWindowLongPtrW", ExactSpelling = true)]
         public static extern uint GetWindowLongPtr(IntPtr hWnd, int nIndex);
 

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -711,6 +711,29 @@ namespace Avalonia.Win32
 
         public void SetEnabled(bool enable) => EnableWindow(_hwnd, enable);
 
+        public IntPtr? ZOrder
+        {
+            get
+            {
+                var lowestHwnd = GetWindow(_hwnd, GetWindowCommand.GW_HWNDLAST);
+
+                var zOrder = 0;
+                var hwnd = lowestHwnd;
+                while (hwnd != IntPtr.Zero)
+                {
+                    if (_hwnd == hwnd)
+                    {
+                        return new IntPtr(zOrder);
+                    }
+
+                    hwnd = GetWindow(hwnd, GetWindowCommand.GW_HWNDPREV);
+                    zOrder++;
+                }
+
+                return null;
+            }
+        }
+
         public void BeginMoveDrag(PointerPressedEventArgs e)
         {
             e.Pointer.Capture(null);