refactor: sub-window's startup location (#2100)

Child window will use current actived window (fall back to `MainWindow`) to calculate the startup location (likes `CenterScreen` mode). The benefits of doing this are:

- Solve the problem that newly opened windows always appear on the primary screen in multi-screen situations.
- Newly opened windows always display on the currently used screen. This reduces the need for users to move the mouse.

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/App.axaml.cs

@@ -148,18 +148,52 @@ public static Task ShowDialog(object data, Window owner = null)
 
         public static void ShowWindow(object data)
         {
-            if (data is Views.ChromelessWindow window)
+            if (data is not Views.ChromelessWindow window)
             {
-                window.Show();
-                return;
-            }
+                window = CreateViewForViewModel(data) as Views.ChromelessWindow;
+                if (window == null)
+                    return;
 
-            window = CreateViewForViewModel(data) as Views.ChromelessWindow;
-            if (window != null)
-            {
                 window.DataContext = data;
-                window.Show();
             }
+
+            do
+            {
+                if (Current?.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime { Windows: { Count: > 0 } windows })
+                {
+                    // Try to find the actived window (fall back to `MainWindow`)
+                    Window actived = windows[0];
+                    if (!actived.IsActive)
+                    {
+                        for (var i = 1; i < windows.Count; i++)
+                        {
+                            var test = windows[i];
+                            if (test.IsActive)
+                            {
+                                actived = test;
+                                break;
+                            }
+                        }
+                    }
+
+                    // Get the screen where current window locates.
+                    var screen = actived.Screens.ScreenFromWindow(actived) ?? actived.Screens.Primary;
+                    if (screen == null)
+                        break;
+
+                    // Calculate the startup position (Center Screen Mode) of target window
+                    var rect = new PixelRect(PixelSize.FromSize(window.ClientSize, actived.DesktopScaling));
+                    var centeredRect = screen.WorkingArea.CenterRect(rect);
+                    if (actived.Screens.ScreenFromPoint(centeredRect.Position) == null)
+                        break;
+
+                    // Use the startup position
+                    window.WindowStartupLocation = WindowStartupLocation.Manual;
+                    window.Position = centeredRect.Position;
+                }
+            } while (false);
+
+            window.Show();
         }
 
         public static async Task<bool> AskConfirmAsync(string message, Models.ConfirmButtonType buttonType = Models.ConfirmButtonType.OkCancel)