fix(claude-do): merge Open diff viewer and merge window as independent top-level w

ClaudeDo-Task: 29564718-d27e-40ad-a61e-9e2b79ede53d

src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs

@@ -161,7 +161,8 @@ public partial class DetailsIslandView : UserControl
                 var window = new DiffViewerView { DataContext = diffVm };
                 window.Closed += (_, _) => _diffViewerWindow = null;
                 _diffViewerWindow = window;
-                window.Show(owner);
+                window.CenterOn(owner);
+                window.Show();   // no owner: an owned window would be pinned above the main window
                 return System.Threading.Tasks.Task.CompletedTask;
             };
 

src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml

@@ -11,7 +11,6 @@
         WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1"
-        WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}">
 
   <Window.KeyBindings>

src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml

@@ -11,7 +11,6 @@
         WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1"
-        WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}">
 
   <Window.KeyBindings>

src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml.cs

@@ -41,7 +41,9 @@ public partial class MergeModalView : Window
             OpenWindows.Remove(vm.TaskId);
             window._closed.TrySetResult();
         };
-        window.Show(owner);
+        // Independent top-level (no owner): an owned window is pinned above its owner by the OS.
+        window.CenterOn(owner);
+        window.Show();
         return window._closed.Task;
     }
 }

src/ClaudeDo.Ui/Views/WindowDialogService.cs

@@ -143,7 +143,8 @@ public sealed class WindowDialogService : IDialogService
             diffVm.CloseAction = () => diffDlg.Close();
             diffDlg.Closed += (_, _) => _diffViewerWindow = null;
             _diffViewerWindow = diffDlg;
-            diffDlg.Show(_owner);
+            diffDlg.CenterOn(_owner);
+            diffDlg.Show();   // no owner: an owned window would be pinned above the main window
         };
         vm.ConfirmAction = message => ConfirmAsync(message);
         if (Shell is { } shell)

src/ClaudeDo.Ui/Views/WindowPlacement.cs

@@ -0,0 +1,28 @@
+using Avalonia;
+using Avalonia.Controls;
+
+namespace ClaudeDo.Ui.Views;
+
+internal static class WindowPlacement
+{
+    /// <summary>
+    /// Centres a not-yet-shown window on <paramref name="owner"/>, clamped to the owner's screen.
+    /// Replaces <c>CenterOwner</c> for windows shown without an owner (which the OS would pin above it).
+    /// </summary>
+    public static void CenterOn(this Window window, Window owner)
+    {
+        var scale = owner.DesktopScaling;
+        var ownerSize = owner.FrameSize ?? owner.ClientSize;
+        var x = owner.Position.X + (int)((ownerSize.Width - window.Width) * scale / 2);
+        var y = owner.Position.Y + (int)((ownerSize.Height - window.Height) * scale / 2);
+
+        if (owner.Screens.ScreenFromWindow(owner)?.WorkingArea is { } area)
+        {
+            x = Math.Max(area.X, Math.Min(x, area.Right - (int)(window.Width * scale)));
+            y = Math.Max(area.Y, Math.Min(y, area.Bottom - (int)(window.Height * scale)));
+        }
+
+        window.WindowStartupLocation = WindowStartupLocation.Manual;
+        window.Position = new PixelPoint(x, y);
+    }
+}