Fix window incorrect positioning with window startup location CenterScreen

samples/Sandbox/MainWindow.axaml

@@ -1,11 +1,4 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
         x:Class="Sandbox.MainWindow">
-
-        <ScrollViewer>
-                <StackPanel>
-                        <Button Margin="0 100000000000000000 0 0">0</Button>
-                        <Button>1</Button>
-                </StackPanel>
-        </ScrollViewer>
 </Window>

src/Avalonia.Controls/Window.cs

@@ -169,6 +169,7 @@ public class Window : WindowBase, IFocusScope, ILayoutRoot
         private readonly Size _maxPlatformClientSize;
         private bool _shown;
         private bool _showingAsDialog;
+        private bool _wasShownBefore;
 
         /// <summary>
         /// Initializes static members of the <see cref="Window"/> class.
@@ -718,6 +719,7 @@ private void ShowCore(Window? owner)
                 StartRendering();
                 PlatformImpl?.Show(ShowActivated, false);
                 OnOpened(EventArgs.Empty);
+                _wasShownBefore = true;
             }
         }
 
@@ -871,6 +873,11 @@ private void OnGotInputWhenDisabled()
 
         private void SetWindowStartupLocation(Window? owner = null)
         {
+            if (_wasShownBefore == true)
+            {
+                return;
+            }
+
             var startupLocation = WindowStartupLocation;
 
             if (startupLocation == WindowStartupLocation.CenterOwner &&

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -513,6 +513,41 @@ public void Hiding_Parent_Window_Should_Close_Dialog_Children()
             }
         }
 
+        [Fact]
+        public void Window_Should_Not_Be_Centered_When_WindowStartupLocation_Is_CenterScreen_And_Window_Is_Hidden_And_Shown()
+        {
+            var screen1 = new Mock<Screen>(1.0, new PixelRect(new PixelSize(1920, 1080)), new PixelRect(new PixelSize(1920, 1040)), true);
+
+            var screens = new Mock<IScreenImpl>();
+            screens.Setup(x => x.AllScreens).Returns(new Screen[] { screen1.Object });
+            screens.Setup(x => x.ScreenFromPoint(It.IsAny<PixelPoint>())).Returns(screen1.Object);
+
+
+            var windowImpl = MockWindowingPlatform.CreateWindowMock();
+            windowImpl.Setup(x => x.ClientSize).Returns(new Size(800, 480));
+            windowImpl.Setup(x => x.DesktopScaling).Returns(1);
+            windowImpl.Setup(x => x.RenderScaling).Returns(1);
+            windowImpl.Setup(x => x.Screen).Returns(screens.Object);
+
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var window = new Window(windowImpl.Object)
+                {
+                    WindowStartupLocation = WindowStartupLocation.CenterScreen
+                };
+
+                window.Show();
+
+                var expected = new PixelPoint(150, 400);
+                window.Position = expected;
+
+                window.IsVisible = false;
+                window.IsVisible = true;
+
+                Assert.Equal(expected, window.Position);
+            }
+        }
+
         [Fact]
         public void Window_Should_Be_Centered_When_WindowStartupLocation_Is_CenterScreen()
         {