diff --git a/src/ClaudeDo.Localization/locales/de.json b/src/ClaudeDo.Localization/locales/de.json
index 74b8eb62..de673041 100644
--- a/src/ClaudeDo.Localization/locales/de.json
+++ b/src/ClaudeDo.Localization/locales/de.json
@@ -365,7 +365,9 @@
       "splitView": "Nebeneinander",
       "wrapLines": "Zeilenumbruch",
       "paneBase": "BASIS",
-      "paneWorktree": "WORKTREE"
+      "paneWorktree": "WORKTREE",
+      "maximize": "Maximieren",
+      "restore": "Wiederherstellen"
     },
     "worktreesOverview": {
       "refresh": "Aktualisieren",
diff --git a/src/ClaudeDo.Localization/locales/en.json b/src/ClaudeDo.Localization/locales/en.json
index 91aba808..d1b70106 100644
--- a/src/ClaudeDo.Localization/locales/en.json
+++ b/src/ClaudeDo.Localization/locales/en.json
@@ -365,7 +365,9 @@
       "splitView": "Side by side",
       "wrapLines": "Wrap lines",
       "paneBase": "BASE",
-      "paneWorktree": "WORKTREE"
+      "paneWorktree": "WORKTREE",
+      "maximize": "Maximize",
+      "restore": "Restore"
     },
     "worktreesOverview": {
       "refresh": "Refresh",
diff --git a/src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml b/src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml
index c3642f82..5bf693bd 100644
--- a/src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml
+++ b/src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml
@@ -14,14 +14,19 @@
                     Background="{DynamicResource DeepBrush}"
                     BorderBrush="{DynamicResource LineBrush}"
                     BorderThickness="0,0,0,1">
-              <Grid ColumnDefinitions="*,Auto" Margin="14,0">
+              <Grid ColumnDefinitions="*,Auto,Auto" Margin="14,0">
                 <TextBlock Text="{TemplateBinding Title}"
                            FontFamily="{DynamicResource MonoFont}"
                            FontSize="{DynamicResource FontSizeMono}"
                            LetterSpacing="1.4"
                            Foreground="{DynamicResource TextBrush}"
                            VerticalAlignment="Center"/>
-                <Button Grid.Column="1" Classes="icon-btn" Content="✕"
+                <Button Name="PART_MaxButton" Grid.Column="1" Classes="icon-btn"
+                        IsVisible="{TemplateBinding ShowMaximizeButton}"
+                        VerticalAlignment="Center">
+                  <PathIcon Name="PART_MaxIcon" Data="{StaticResource Icon.WinMax}" Width="10" Height="10"/>
+                </Button>
+                <Button Grid.Column="2" Classes="icon-btn" Content="✕"
                         FontSize="{DynamicResource FontSizeBody}"
                         Command="{TemplateBinding CloseCommand}"
                         VerticalAlignment="Center"/>
diff --git a/src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml.cs b/src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml.cs
index 19b17b2a..43e49d12 100644
--- a/src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml.cs
+++ b/src/ClaudeDo.Ui/Views/Controls/ModalShell.axaml.cs
@@ -3,6 +3,8 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
+using Avalonia.Interactivity;
+using Avalonia.Media;
 
 namespace ClaudeDo.Ui.Views.Controls;
 
@@ -18,17 +20,29 @@ public class ModalShell : ContentControl
     public static readonly StyledProperty<ICommand?> CloseCommandProperty =
         AvaloniaProperty.Register<ModalShell, ICommand?>(nameof(CloseCommand));
 
+    // Opt-in: most modals are fixed-purpose dialogs that don't need maximise. Set true only
+    // on windows (e.g. the diff viewer) that benefit from a maximised full-screen layout.
+    public static readonly StyledProperty<bool> ShowMaximizeButtonProperty =
+        AvaloniaProperty.Register<ModalShell, bool>(nameof(ShowMaximizeButton));
+
     public string? Title { get => GetValue(TitleProperty); set => SetValue(TitleProperty, value); }
     public object? Footer { get => GetValue(FooterProperty); set => SetValue(FooterProperty, value); }
     public ICommand? CloseCommand { get => GetValue(CloseCommandProperty); set => SetValue(CloseCommandProperty, value); }
+    public bool ShowMaximizeButton { get => GetValue(ShowMaximizeButtonProperty); set => SetValue(ShowMaximizeButtonProperty, value); }
 
     private Window? _window;
+    private PathIcon? _maxIcon;
+    private Button? _maxButton;
 
     protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
     {
         base.OnApplyTemplate(e);
         if (e.NameScope.Find<Border>("PART_TitleBar") is { } bar)
             bar.PointerPressed += OnTitleBarPressed;
+        _maxButton = e.NameScope.Find<Button>("PART_MaxButton");
+        if (_maxButton is not null) _maxButton.Click += OnMaxButtonClick;
+        _maxIcon = e.NameScope.Find<PathIcon>("PART_MaxIcon");
+        UpdateMaxIcon();
     }
 
     /// Mirrors what MainWindow does for itself: with an extended client area a maximised window
@@ -42,6 +56,7 @@ public class ModalShell : ContentControl
         if (_window is null) return;
         Margin = _window.OffScreenMargin;
         _window.PropertyChanged += OnWindowPropertyChanged;
+        UpdateMaxIcon();
     }
 
     protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
@@ -55,6 +70,32 @@ public class ModalShell : ContentControl
     {
         if (e.Property == Window.OffScreenMarginProperty && _window is not null)
             Margin = _window.OffScreenMargin;
+        if (e.Property == Window.WindowStateProperty)
+            UpdateMaxIcon();
+    }
+
+    private void UpdateMaxIcon()
+    {
+        if (_maxIcon is null || _window is null) return;
+        var maximized = _window.WindowState == WindowState.Maximized;
+        var key = maximized ? "Icon.WinRestore" : "Icon.WinMax";
+        if (this.TryFindResource(key, out var geometry) && geometry is Geometry g)
+            _maxIcon.Data = g;
+        if (_maxButton is not null)
+        {
+            var tipKey = maximized ? "modals.diff.restore" : "modals.diff.maximize";
+            ToolTip.SetTip(_maxButton, ClaudeDo.Ui.Localization.Loc.T(tipKey));
+        }
+    }
+
+    private void OnMaxButtonClick(object? sender, RoutedEventArgs e) => ToggleMaximize();
+
+    private void ToggleMaximize()
+    {
+        if (_window is null) return;
+        _window.WindowState = _window.WindowState == WindowState.Maximized
+            ? WindowState.Normal
+            : WindowState.Maximized;
     }
 
     /// <summary>
@@ -74,6 +115,11 @@ public class ModalShell : ContentControl
     private void OnTitleBarPressed(object? sender, PointerPressedEventArgs e)
     {
         if (!e.GetCurrentPoint(this).Properties.IsLeftButtonPressed) return;
+        if (ShowMaximizeButton && e.ClickCount == 2)
+        {
+            ToggleMaximize();
+            return;
+        }
         (TopLevel.GetTopLevel(this) as Window)?.BeginMoveDrag(e);
     }
 }
diff --git a/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs b/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs
index 52f95286..4eaa90c2 100644
--- a/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs
+++ b/src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml.cs
@@ -18,6 +18,7 @@ public partial class DetailsIslandView : UserControl
     // never dragged stay dynamic (Auto-sized description).
     private readonly Dictionary<string, double> _descriptionHeights = new();
     private DetailsIslandViewModel? _vm;
+    private DiffViewerView? _diffViewerWindow;
 
     public DetailsIslandView()
     {
@@ -142,12 +143,26 @@ public partial class DetailsIslandView : UserControl
             vm.PropertyChanged += OnViewModelPropertyChanged;
             ApplyResizeStateForCurrentTask();
 
-            vm.Merge.ShowDiffViewer = async (diffVm) =>
+            // Non-modal: the diff viewer must not block navigating the rest of the app.
+            // Reuse the one open window (swap its DataContext + bring to front) instead of
+            // stacking a second one when the user re-opens the diff for another task.
+            vm.Merge.ShowDiffViewer = (diffVm) =>
             {
                 var owner = TopLevel.GetTopLevel(this) as Window;
-                if (owner == null) return;
-                var modal = new DiffViewerView { DataContext = diffVm };
-                await modal.ShowDialog(owner);
+                if (owner == null) return System.Threading.Tasks.Task.CompletedTask;
+                if (_diffViewerWindow is { } existing)
+                {
+                    existing.DataContext = diffVm;
+                    if (existing.WindowState == WindowState.Minimized)
+                        existing.WindowState = WindowState.Normal;
+                    existing.Activate();
+                    return System.Threading.Tasks.Task.CompletedTask;
+                }
+                var window = new DiffViewerView { DataContext = diffVm };
+                window.Closed += (_, _) => _diffViewerWindow = null;
+                _diffViewerWindow = window;
+                window.Show(owner);
+                return System.Threading.Tasks.Task.CompletedTask;
             };
 
             vm.Merge.ShowMergeModal = async (mergeVm) =>
diff --git a/src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml b/src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml
index 89fbd4e1..2788b78a 100644
--- a/src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml
+++ b/src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml
@@ -18,7 +18,7 @@
     <KeyBinding Gesture="Escape" Command="{Binding CloseCommand}"/>
   </Window.KeyBindings>
 
-  <ctl:ModalShell Title="{loc:Tr modals.diff.title}" CloseCommand="{Binding CloseCommand}">
+  <ctl:ModalShell Title="{loc:Tr modals.diff.title}" CloseCommand="{Binding CloseCommand}" ShowMaximizeButton="True">
     <ctl:ModalShell.Footer>
       <StackPanel Orientation="Horizontal" Spacing="8"
                   HorizontalAlignment="Right" VerticalAlignment="Center">
diff --git a/src/ClaudeDo.Ui/Views/WindowDialogService.cs b/src/ClaudeDo.Ui/Views/WindowDialogService.cs
index 96a826af..fdf7744d 100644
--- a/src/ClaudeDo.Ui/Views/WindowDialogService.cs
+++ b/src/ClaudeDo.Ui/Views/WindowDialogService.cs
@@ -27,6 +27,7 @@ public sealed class WindowDialogService : IDialogService
 {
     private readonly Window _owner;
     private MissionControlWindow? _missionControl;
+    private DiffViewerView? _diffViewerWindow;
 
     public WindowDialogService(Window owner) => _owner = owner;
 
@@ -108,10 +109,23 @@ public sealed class WindowDialogService : IDialogService
         };
         vm.ShowDiffAction = diffVm =>
         {
+            _ = diffVm.LoadAsync();
+            // Non-modal: must not block the (still-open) worktrees overview or the main
+            // window behind it. Reuse the one open window rather than stacking a second.
+            if (_diffViewerWindow is { } existing)
+            {
+                existing.DataContext = diffVm;
+                diffVm.CloseAction = () => existing.Close();
+                if (existing.WindowState == WindowState.Minimized)
+                    existing.WindowState = WindowState.Normal;
+                existing.Activate();
+                return;
+            }
             var diffDlg = new DiffViewerView { DataContext = diffVm };
             diffVm.CloseAction = () => diffDlg.Close();
-            _ = diffVm.LoadAsync();
-            _ = diffDlg.ShowDialog(_owner);
+            diffDlg.Closed += (_, _) => _diffViewerWindow = null;
+            _diffViewerWindow = diffDlg;
+            diffDlg.Show(_owner);
         };
         vm.ConfirmAction = ConfirmAsync;
         if (Shell is { } shell)