diff --git a/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs b/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs
index 671c456e..cd7dc617 100644
--- a/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs
+++ b/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs
@@ -165,12 +165,12 @@ public partial class DetailsIslandView : UserControl
                 return System.Threading.Tasks.Task.CompletedTask;
             };
 
-            vm.Merge.ShowMergeModal = async (mergeVm) =>
+            // Non-modal so the main window stays usable while the merge + verify runs; the task
+            // completes on close so the diff viewer can still react to the outcome.
+            vm.Merge.ShowMergeModal = (mergeVm) =>
             {
                 var owner = TopLevel.GetTopLevel(this) as Window;
-                if (owner == null) return;
-                var modal = new MergeModalView { DataContext = mergeVm };
-                await modal.ShowDialog(owner);
+                return owner == null ? System.Threading.Tasks.Task.CompletedTask : MergeModalView.ShowAsync(owner, mergeVm);
             };
 
         }
diff --git a/src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml.cs b/src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml.cs
index c52fcb25..8999811d 100644
--- a/src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml.cs
+++ b/src/ClaudeDo.Ui/Views/Modals/MergeModalView.axaml.cs
@@ -5,6 +5,11 @@ namespace ClaudeDo.Ui.Views.Modals;
 
 public partial class MergeModalView : Window
 {
+    /// Open merge windows by task id — the guard against starting the same merge twice.
+    private static readonly Dictionary<string, MergeModalView> OpenWindows = new();
+
+    private readonly TaskCompletionSource _closed = new();
+
     public MergeModalView()
     {
         InitializeComponent();
@@ -17,5 +22,26 @@ public partial class MergeModalView : Window
         };
     }
 
-
+    /// Shows the merge window non-modally so the owner stays usable while the merge (and its verify
+    /// command) runs. Completes when the window closes; a second call for a task whose window is
+    /// already open activates it and waits on that one instead of opening another.
+    public static Task ShowAsync(Window owner, MergeModalViewModel vm)
+    {
+        if (OpenWindows.TryGetValue(vm.TaskId, out var existing))
+        {
+            if (existing.WindowState == WindowState.Minimized)
+                existing.WindowState = WindowState.Normal;
+            existing.Activate();
+            return existing._closed.Task;
+        }
+        var window = new MergeModalView { DataContext = vm };
+        OpenWindows[vm.TaskId] = window;
+        window.Closed += (_, _) =>
+        {
+            OpenWindows.Remove(vm.TaskId);
+            window._closed.TrySetResult();
+        };
+        window.Show(owner);
+        return window._closed.Task;
+    }
 }
diff --git a/src/ClaudeDo.Ui/Views/WindowDialogService.cs b/src/ClaudeDo.Ui/Views/WindowDialogService.cs
index d9b1ca10..60503a3f 100644
--- a/src/ClaudeDo.Ui/Views/WindowDialogService.cs
+++ b/src/ClaudeDo.Ui/Views/WindowDialogService.cs
@@ -149,11 +149,7 @@ public sealed class WindowDialogService : IDialogService
         if (Shell is { } shell)
         {
             vm.ResolveMergeVm = shell.ResolveMergeVm;
-            vm.ShowMergeAction = async mergeVm =>
-            {
-                var mergeDlg = new MergeModalView { DataContext = mergeVm };
-                await mergeDlg.ShowDialog(_owner);
-            };
+            vm.ShowMergeAction = mergeVm => MergeModalView.ShowAsync(_owner, mergeVm);
         }
         await dlg.ShowDialog(_owner);
     }
diff --git a/tests/ClaudeDo.Ui.Tests/ViewModels/DiffViewerViewModelTests.cs b/tests/ClaudeDo.Ui.Tests/ViewModels/DiffViewerViewModelTests.cs
index 105b9400..0e21dee6 100644
--- a/tests/ClaudeDo.Ui.Tests/ViewModels/DiffViewerViewModelTests.cs
+++ b/tests/ClaudeDo.Ui.Tests/ViewModels/DiffViewerViewModelTests.cs
@@ -335,4 +335,30 @@ public class DiffViewerViewModelTests
         Assert.Contains("planning task not found", vm.CombinedWarning);
         Assert.Equal("", vm.DisplayedDiff);
     }
+
+    // ── Merge button: the modal is non-modal, the command spans its lifetime ──
+
+    [Fact]
+    public async Task Merge_WhileMergeWindowOpen_DoesNotStartSecondMerge()
+    {
+        var worker = new FakePlanningWorker();
+        var vm = new DiffViewerViewModel(null!, worker, TestSettings());
+        vm.ConfigureWorktree("/wt", "main", "task-1", "Title");
+        var windowClosed = new TaskCompletionSource();
+        var resolved = 0;
+        var shown = 0;
+        vm.ResolveMergeVm = () => { resolved++; return new MergeModalViewModel(worker, new MergeCoordinator()); };
+        vm.ShowMergeModal = _ => { shown++; return windowClosed.Task; };
+
+        var first = vm.MergeCommand.ExecuteAsync(null);
+        while (shown == 0) await Task.Delay(10);
+
+        Assert.False(vm.MergeCommand.CanExecute(null));
+        Assert.Equal(1, resolved);
+        Assert.Equal(1, shown);
+
+        windowClosed.SetResult();
+        await first;
+        Assert.True(vm.MergeCommand.CanExecute(null));
+    }
 }