Use Screen API.

src/Avalonia.Controls/Screens.cs

@@ -39,7 +39,7 @@ namespace Avalonia.Controls
             return currMaxScreen;
         }
         
-        public Screen SceenFromPoint(Point point)
+        public Screen ScreenFromPoint(Point point)
         {
             return All.FirstOrDefault(x=>x.Bounds.Contains(point));        
         }

src/Avalonia.Controls/Window.cs

@@ -376,13 +376,10 @@ namespace Avalonia.Controls
         {
             if (WindowStartupLocation == WindowStartupLocation.CenterScreen)
             {
-                // This should be using a Screen API, but we don't have one yet and
-                // PlatformImpl.MaxClientSize is the best we have.
-                if (PlatformImpl != null)
-                {
-                    var positionAsSize = PlatformImpl.MaxClientSize / 2 - ClientSize / 2;
-                    Position = new Point(positionAsSize.Width, positionAsSize.Height);
-                }
+                var screen = Screens.ScreenFromPoint(Bounds.Position);
+
+                if (screen != null)
+                    Position = screen.WorkingArea.CenterRect(new Rect(ClientSize)).Position;
             }
             else if (WindowStartupLocation == WindowStartupLocation.CenterOwner)
             {

src/Windows/Avalonia.Win32/ScreenImpl.cs

@@ -41,8 +41,8 @@ namespace Avalonia.Win32
                                 Rect avaloniaBounds = new Rect(bounds.left, bounds.top, bounds.right - bounds.left,
                                     bounds.bottom - bounds.top);
                                 Rect avaloniaWorkArea =
-                                    new Rect(workingArea.left, workingArea.top, workingArea.right - bounds.left,
-                                        workingArea.bottom - bounds.top);
+                                    new Rect(workingArea.left, workingArea.top, workingArea.right - workingArea.left,
+                                        workingArea.bottom - workingArea.top);
                                 screens[index] =
                                     new WinScreen(avaloniaBounds, avaloniaWorkArea, monitorInfo.dwFlags == 1,
                                         monitor);

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -239,32 +239,38 @@ namespace Avalonia.Controls.UnitTests
         }
 
         [Fact]
-        public void Window_Should_Be_Centered_When_Window_Startup_Location_Is_Center_Screen()
+        public void Window_Should_Be_Centered_When_WindowStartupLocation_Is_CenterScreen()
         {
+            var screen1 = new Mock<Screen>(new Rect(new Size(1920, 1080)), new Rect(new Size(1920, 1040)), true);
+            var screen2 = new Mock<Screen>(new Rect(new Size(1366, 768)), new Rect(new Size(1366, 728)), false);
+
+            var screens = new Mock<IScreenImpl>();
+            screens.Setup(x => x.AllScreens).Returns(new Screen[] { screen1.Object, screen2.Object });
+
             var windowImpl = new Mock<IWindowImpl>();
             windowImpl.SetupProperty(x => x.Position);
             windowImpl.Setup(x => x.ClientSize).Returns(new Size(800, 480));
-            windowImpl.Setup(x => x.MaxClientSize).Returns(new Size(1920, 1080));
             windowImpl.Setup(x => x.Scaling).Returns(1);
+            windowImpl.Setup(x => x.Screen).Returns(screens.Object);
 
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
-                var window = new Window();
+                var window = new Window(windowImpl.Object);
                 window.WindowStartupLocation = WindowStartupLocation.CenterScreen;
                 window.Position = new Point(60, 40);
 
                 window.Show();
 
                 var expectedPosition = new Point(
-                    window.PlatformImpl.MaxClientSize.Width / 2 - window.ClientSize.Width / 2,
-                    window.PlatformImpl.MaxClientSize.Height / 2 - window.ClientSize.Height / 2);
+                    screen1.Object.WorkingArea.Size.Width / 2 - window.ClientSize.Width / 2,
+                    screen1.Object.WorkingArea.Size.Height / 2 - window.ClientSize.Height / 2);
 
                 Assert.Equal(window.Position, expectedPosition);
             }
         }
 
         [Fact]
-        public void Window_Should_Be_Centered_Relative_To_Owner_When_Window_Startup_Location_Is_Center_Owner()
+        public void Window_Should_Be_Centered_Relative_To_Owner_When_WindowStartupLocation_Is_CenterOwner()
         {
             var parentWindowImpl = new Mock<IWindowImpl>();
             parentWindowImpl.SetupProperty(x => x.Position);