Fall back to CenterScreen when owner is minimized.

When showing a child window with `WindowStartupLocation = CenterOwner` to a minimized owner window, we were displaying the child in an invalid position. Use the WPF behavior of falling back to `CenterScreen` in this situation.

Fixes #7582

src/Avalonia.Controls/Window.cs

@@ -858,6 +858,17 @@ namespace Avalonia.Controls
 
         private void SetWindowStartupLocation(IWindowBaseImpl? owner = null)
         {
+            var startupLocation = WindowStartupLocation;
+
+            if (startupLocation == WindowStartupLocation.CenterOwner &&
+                Owner is Window ownerWindow &&
+                ownerWindow.WindowState == WindowState.Minimized)
+            {
+                // If startup location is CenterOwner, but owner is minimized then fall back
+                // to CenterScreen. This behavior is consistent with WPF.
+                startupLocation = WindowStartupLocation.CenterScreen;
+            }
+
             var scaling = owner?.DesktopScaling ?? PlatformImpl?.DesktopScaling ?? 1;
 
             // TODO: We really need non-client size here.
@@ -865,7 +876,7 @@ namespace Avalonia.Controls
                 PixelPoint.Origin,
                 PixelSize.FromSize(ClientSize, scaling));
 
-            if (WindowStartupLocation == WindowStartupLocation.CenterScreen)
+            if (startupLocation == WindowStartupLocation.CenterScreen)
             {
                 var screen = Screens.ScreenFromPoint(owner?.Position ?? Position);
 
@@ -874,7 +885,7 @@ namespace Avalonia.Controls
                     Position = screen.WorkingArea.CenterRect(rect).Position;
                 }
             }
-            else if (WindowStartupLocation == WindowStartupLocation.CenterOwner)
+            else if (startupLocation == WindowStartupLocation.CenterOwner)
             {
                 if (owner != null)
                 {