fix opening child window center screen when parent maximized.

src/Avalonia.Controls/Platform/IScreenImpl.cs

@@ -7,5 +7,7 @@ namespace Avalonia.Platform
         int ScreenCount { get; }
 
         IReadOnlyList<Screen> AllScreens { get; }
+
+        Screen? ScreenFromWindow(IWindowImpl windowImpl);
     }
 }

src/Avalonia.Controls/Screens.cs

@@ -52,5 +52,22 @@ namespace Avalonia.Controls
             var br = visual.PointToScreen(visual.Bounds.BottomRight);
             return ScreenFromBounds(new PixelRect(tl, br));
         }
+
+        public Screen? ScreenFromWindow(Window window)
+        {
+            Screen? screen = null;
+            
+            if (window.PlatformImpl != null)
+            {
+                 screen = _iScreenImpl?.ScreenFromWindow(window.PlatformImpl) ?? null;
+
+                 if (screen is { })
+                 {
+                     return screen;
+                 }
+            }
+
+            return ScreenFromPoint(window.Position);
+        }
     }
 }

src/Avalonia.Controls/Window.cs

@@ -879,8 +879,17 @@ namespace Avalonia.Controls
 
             if (startupLocation == WindowStartupLocation.CenterScreen)
             {
-                var screen = Screens.ScreenFromPoint(owner?.Position ?? Position);
-
+                Screen? screen = null;
+                
+                if (Owner is Window ownerWin)
+                {
+                    screen = Screens.ScreenFromWindow(ownerWin);
+                }
+                else
+                {
+                    screen = Screens.ScreenFromPoint(Position);
+                }
+                
                 if (screen != null)
                 {
                     Position = screen.WorkingArea.CenterRect(rect).Position;

src/Avalonia.DesignerSupport/Remote/Stubs.cs

@@ -237,5 +237,10 @@ namespace Avalonia.DesignerSupport.Remote
 
         public IReadOnlyList<Screen> AllScreens { get; } =
             new Screen[] { new Screen(1, new PixelRect(0, 0, 4000, 4000), new PixelRect(0, 0, 4000, 4000), true) };
+        
+        public Screen? ScreenFromWindow(IWindowImpl windowImpl)
+        {
+            return null;
+        }
     }
 }

src/Avalonia.Headless/HeadlessPlatformStubs.cs

@@ -200,5 +200,10 @@ namespace Avalonia.Headless
             new Screen(1, new PixelRect(0, 0, 1920, 1280),
                 new PixelRect(0, 0, 1920, 1280), true),
         };
+        
+        public Screen ScreenFromWindow(IWindowImpl windowImpl)
+        {
+            return null;
+        }
     }
 }

src/Avalonia.Native/ScreenImpl.cs

@@ -42,6 +42,11 @@ namespace Avalonia.Native
                 return Array.Empty<Screen>();
             }
         }
+        
+        public Screen ScreenFromWindow(IWindowImpl windowImpl)
+        {
+            return null;
+        }
 
         public void Dispose ()
         {

src/Avalonia.X11/X11Screens.cs

@@ -17,6 +17,11 @@ namespace Avalonia.X11
         {
             _impl = impl;
         }
+        
+        public Screen ScreenFromWindow(IWindowImpl windowImpl)
+        {
+            return null;
+        }
 
         static unsafe X11Screen[] UpdateWorkArea(X11Info info, X11Screen[] screens)
         {

src/Windows/Avalonia.Win32/ScreenImpl.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Avalonia.Platform;
 using Avalonia.Win32.Interop;
 using static Avalonia.Win32.Interop.UnmanagedMethods;
@@ -70,5 +71,13 @@ namespace Avalonia.Win32
         {
             _allScreens = null;
         }
+
+        public Screen? ScreenFromWindow(IWindowImpl windowImpl)
+        {
+            var hmonitor = MonitorFromWindow(windowImpl.Handle.Handle, MONITOR.MONITOR_DEFAULTTONEAREST);
+
+            return AllScreens.Select(x => x as WinScreen).Where(x => x is { })
+                .FirstOrDefault(x => x.Handle == hmonitor);
+        }
     }
 }

src/Windows/Avalonia.Win32/WinScreen.cs

@@ -21,5 +21,7 @@ namespace Avalonia.Win32
         {
             return (obj is WinScreen screen) ? this._hMonitor == screen._hMonitor : base.Equals(obj);
         }
+
+        internal IntPtr Handle => _hMonitor;
     }
 }