diff --git a/docs/explore-notes/review-merge.md b/docs/explore-notes/review-merge.md
index b01365f9..775f055a 100644
--- a/docs/explore-notes/review-merge.md
+++ b/docs/explore-notes/review-merge.md
@@ -1,7 +1,9 @@
 # Review, merge & conflict resolution
 
 > **Explore-note — verify before trusting.** Distilled map of a subsystem, not authoritative.
-> Last verified against commit `0d1e3b9` (2026-08-06).
+> Last verified against branch `worktree-diff-side-by-side` @ 2026-08-07, the commit right after
+> `cf80fe3` that reworks Planning mode and retires `DiffLinesView` (exact SHA not known at write
+> time — see `git log` on that branch).
 > Drift check: `git log --oneline 20bce9b..HEAD -- src/ClaudeDo.Worker/Lifecycle src/ClaudeDo.Worker/State src/ClaudeDo.Worker/Planning src/ClaudeDo.Ui/ViewModels/Conflicts src/ClaudeDo.Worker/External`
 > Stable structure only (no line numbers). See docs/explore-notes/README.md.
 
@@ -219,15 +221,20 @@ Review **Approve** on conflict, and the **Merge** button in the Diff window (a c
 ## Diff stack (UI)
 
 `UnifiedDiffParser` (static) parses `git diff` output into `DiffFileViewModel`s, detecting
-added/deleted/renamed/binary files and per-line numbers; `Flatten` injects file-header rows for
-a combined single-pane view. `DiffModels.cs` holds the shared types (`DiffLineViewModel`,
-`DiffFileViewModel`, `DiffLineKind`, `DiffFileStatus`, `SubtaskDiffRow`,
+added/deleted/renamed/binary files and per-line numbers. `DiffModels.cs` holds the shared types
+(`DiffLineViewModel`, `DiffFileViewModel`, `DiffLineKind`, `DiffFileStatus`, `SubtaskDiffRow`,
 `DiffTreeNodeViewModel`, `DiffTree`).
 
 `DiffViewerViewModel` is one unified read-only viewer with two modes:
 - **Files** — dirty worktree / branch-vs-base / commit-range. Loads via `GitService`, folder
   file-tree left + per-file diff pane right, Merge button for a live branch source.
-- **Planning** — per-subtask diffs via `GetPlanningAggregateAsync`, subtask list left + flat
-  diff right, combined integration-branch toggle.
+- **Planning** — per-subtask diffs via `GetPlanningAggregateAsync`, subtask list left + one
+  editor per file right (`PlanningFiles`), combined integration-branch toggle.
 
-`DiffLinesView` renders per-file content with binary/empty placeholders.
+Both modes render through `DiffAlignment` (pure — pairs diff lines into side-by-side rows and
+computes word-diff spans) and `DiffTextView` (AvaloniaEdit + TextMate highlighting keyed off the
+file extension, unified/split layout, optional line wrap, synced scrolling). Files mode hosts one
+`DiffTextView` for the selected file; Planning mode hosts one per file in `PlanningFiles` so each
+gets its own grammar (one editor can only carry one TextMate grammar). The split/wrap toggles
+persist to `ui.config.json` via `AppSettings` and reach the per-file editors in Planning mode
+too.
diff --git a/src/ClaudeDo.Ui/CLAUDE.md b/src/ClaudeDo.Ui/CLAUDE.md
index 542e0688..e06e38f0 100644
--- a/src/ClaudeDo.Ui/CLAUDE.md
+++ b/src/ClaudeDo.Ui/CLAUDE.md
@@ -31,7 +31,7 @@ ViewModels/
   Conflicts/  — ConflictResolverViewModel + ConflictModels
 Views/          — mirrors the VM layout; Islands/Detail/ holds TaskHeaderBar,
                   DescriptionStepsCard, WorkConsole; plus SessionTerminalView
-Views/Controls/ — MarkdownView, ModalShell, ThemedDatePicker, DiffLinesView, InheritedBadge,
+Views/Controls/ — MarkdownView, ModalShell, ThemedDatePicker, DiffTextView, InheritedBadge,
                   AgentConfigEditor
 Design/         — Tokens.axaml (design tokens; merged before styles)
                   + IslandStyles.axaml (component styles + the filled icon geometry library)
@@ -68,7 +68,10 @@ warn/error filter), `WorkerConnectionModalViewModel`, `AboutModalViewModel`.
 ## Diff & Conflicts
 
 `UnifiedDiffParser` (static) + `DiffModels.cs` shared types + `DiffViewerViewModel` (one unified
-read-only viewer, Files and Planning modes) + `DiffLinesView`.
+read-only viewer, Files and Planning modes) render through `DiffAlignment` (pure — pairs diff
+lines into side-by-side rows and computes word-diff spans) and `DiffTextView` (AvaloniaEdit +
+TextMate highlighting, unified/split layout, optional line wrap, synced scrolling). The
+split/wrap toggles persist to `ui.config.json` via `AppSettings`.
 `ConflictResolverViewModel` is an in-app Rider-style 3-pane AvaloniaEdit merge editor for both
 single-task and planning unit-merge conflicts. Full detail →
 [review-merge](../../docs/explore-notes/review-merge.md).
diff --git a/src/ClaudeDo.Ui/ViewModels/Modals/DiffModels.cs b/src/ClaudeDo.Ui/ViewModels/Modals/DiffModels.cs
index 9f9dd0b5..b49570cc 100644
--- a/src/ClaudeDo.Ui/ViewModels/Modals/DiffModels.cs
+++ b/src/ClaudeDo.Ui/ViewModels/Modals/DiffModels.cs
@@ -3,7 +3,7 @@ using CommunityToolkit.Mvvm.ComponentModel;
 
 namespace ClaudeDo.Ui.ViewModels.Modals;
 
-// Shared diff models used by UnifiedDiffParser, DiffLinesView and DiffViewerViewModel.
+// Shared diff models used by UnifiedDiffParser, DiffTextView and DiffViewerViewModel.
 
 public enum DiffLineKind { Add, Del, Ctx, File }
 
diff --git a/src/ClaudeDo.Ui/ViewModels/Modals/DiffViewerViewModel.cs b/src/ClaudeDo.Ui/ViewModels/Modals/DiffViewerViewModel.cs
index 455e1be4..ef11ff9b 100644
--- a/src/ClaudeDo.Ui/ViewModels/Modals/DiffViewerViewModel.cs
+++ b/src/ClaudeDo.Ui/ViewModels/Modals/DiffViewerViewModel.cs
@@ -56,7 +56,8 @@ public sealed partial class DiffViewerViewModel : ViewModelBase
 
     // ── Right pane ──────────────────────────────────────────────────────────
     [ObservableProperty] private DiffFileViewModel? _selectedFile;       // Files mode
-    public ObservableCollection<DiffLineViewModel> DiffLines { get; } = new(); // Planning mode
+    // Planning mode: one entry per file so each gets its own editor and grammar.
+    public ObservableCollection<DiffFileViewModel> PlanningFiles { get; } = new();
     [ObservableProperty] private string _displayedDiff = "";
     [ObservableProperty] private string? _statusMessage;
 
@@ -250,9 +251,9 @@ public sealed partial class DiffViewerViewModel : ViewModelBase
 
     partial void OnDisplayedDiffChanged(string value)
     {
-        DiffLines.Clear();
-        foreach (var line in UnifiedDiffParser.Flatten(UnifiedDiffParser.Parse(value)))
-            DiffLines.Add(line);
+        PlanningFiles.Clear();
+        foreach (var file in UnifiedDiffParser.Parse(value))
+            PlanningFiles.Add(file);
     }
 
     // ── Merge (Files mode, branch source) ───────────────────────────────────
diff --git a/src/ClaudeDo.Ui/ViewModels/Modals/UnifiedDiffParser.cs b/src/ClaudeDo.Ui/ViewModels/Modals/UnifiedDiffParser.cs
index bc49d9b5..9cecf0e1 100644
--- a/src/ClaudeDo.Ui/ViewModels/Modals/UnifiedDiffParser.cs
+++ b/src/ClaudeDo.Ui/ViewModels/Modals/UnifiedDiffParser.cs
@@ -132,21 +132,6 @@ public static class UnifiedDiffParser
         return files;
     }
 
-    /// Flattens multiple parsed files into a single line stream, inserting a
-    /// file-header row before each file so boundaries are visible in a
-    /// single-pane (combined) view.
-    public static List<DiffLineViewModel> Flatten(IEnumerable<DiffFileViewModel> files)
-    {
-        var lines = new List<DiffLineViewModel>();
-        foreach (var file in files)
-        {
-            lines.Add(new DiffLineViewModel { Kind = DiffLineKind.File, Text = file.Path });
-            foreach (var line in file.Lines)
-                lines.Add(line);
-        }
-        return lines;
-    }
-
     private static void ParseHunkHeader(string header, out int oldStart, out int newStart)
     {
         oldStart = 1; newStart = 1;
diff --git a/src/ClaudeDo.Ui/Views/Controls/DiffLinesView.axaml b/src/ClaudeDo.Ui/Views/Controls/DiffLinesView.axaml
deleted file mode 100644
index d2149b04..00000000
--- a/src/ClaudeDo.Ui/Views/Controls/DiffLinesView.axaml
+++ /dev/null
@@ -1,82 +0,0 @@
-<UserControl xmlns="https://github.com/avaloniaui"
-             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-             xmlns:vm="using:ClaudeDo.Ui.ViewModels.Modals"
-             x:Class="ClaudeDo.Ui.Views.Controls.DiffLinesView"
-             x:Name="Root">
-
-  <UserControl.Styles>
-    <!-- diff line row tints via Tag selector (compiled-binding-friendly) -->
-    <Style Selector="Border.diff-line[Tag=add]">
-      <Setter Property="Background" Value="{StaticResource RunningTintBrush}"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=del]">
-      <Setter Property="Background" Value="{StaticResource ErrorTintBrush}"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=ctx]">
-      <Setter Property="Background" Value="Transparent"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=file]">
-      <Setter Property="Background" Value="{StaticResource Surface3Brush}"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=add] TextBlock.diff-sign">
-      <Setter Property="Foreground" Value="{StaticResource MossBrightBrush}"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=del] TextBlock.diff-sign">
-      <Setter Property="Foreground" Value="{StaticResource BloodBrush}"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=ctx] TextBlock.diff-sign">
-      <Setter Property="Foreground" Value="{StaticResource TextFaintBrush}"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=add] TextBlock.diff-text">
-      <Setter Property="Foreground" Value="{StaticResource MossBrightBrush}"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=del] TextBlock.diff-text">
-      <Setter Property="Foreground" Value="{StaticResource BloodBrush}"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=ctx] TextBlock.diff-text">
-      <Setter Property="Foreground" Value="{StaticResource TextDimBrush}"/>
-    </Style>
-    <Style Selector="Border.diff-line[Tag=file] TextBlock.diff-text">
-      <Setter Property="Foreground" Value="{StaticResource TextBrush}"/>
-      <Setter Property="FontWeight" Value="SemiBold"/>
-    </Style>
-  </UserControl.Styles>
-
-  <ItemsControl ItemsSource="{Binding #Root.Lines}">
-    <ItemsControl.ItemTemplate>
-      <DataTemplate x:DataType="vm:DiffLineViewModel">
-        <Border Classes="diff-line"
-                Tag="{Binding ClassName}"
-                Padding="4,1">
-          <Grid ColumnDefinitions="48,48,16,*">
-            <!-- Old line number -->
-            <TextBlock Grid.Column="0"
-                       Text="{Binding OldNo}"
-                       Classes="diff-lineno"
-                       HorizontalAlignment="Right"
-                       Margin="0,0,8,0"/>
-            <!-- New line number -->
-            <TextBlock Grid.Column="1"
-                       Text="{Binding NewNo}"
-                       Classes="diff-lineno"
-                       HorizontalAlignment="Right"
-                       Margin="0,0,8,0"/>
-            <!-- Sign -->
-            <TextBlock Grid.Column="2"
-                       Classes="diff-sign"
-                       Text="{Binding Sign}"
-                       FontFamily="{DynamicResource MonoFont}"
-                       FontSize="{StaticResource FontSizeMono}"/>
-            <!-- Line text -->
-            <TextBlock Grid.Column="3"
-                       Classes="diff-text"
-                       Text="{Binding Text}"
-                       FontFamily="{DynamicResource MonoFont}"
-                       FontSize="{StaticResource FontSizeMono}"
-                       TextWrapping="NoWrap"/>
-          </Grid>
-        </Border>
-      </DataTemplate>
-    </ItemsControl.ItemTemplate>
-  </ItemsControl>
-</UserControl>
diff --git a/src/ClaudeDo.Ui/Views/Controls/DiffLinesView.axaml.cs b/src/ClaudeDo.Ui/Views/Controls/DiffLinesView.axaml.cs
deleted file mode 100644
index 340541c3..00000000
--- a/src/ClaudeDo.Ui/Views/Controls/DiffLinesView.axaml.cs
+++ /dev/null
@@ -1,19 +0,0 @@
-using System.Collections;
-using Avalonia;
-using Avalonia.Controls;
-
-namespace ClaudeDo.Ui.Views.Controls;
-
-public partial class DiffLinesView : UserControl
-{
-    public static readonly StyledProperty<IEnumerable?> LinesProperty =
-        AvaloniaProperty.Register<DiffLinesView, IEnumerable?>(nameof(Lines));
-
-    public IEnumerable? Lines
-    {
-        get => GetValue(LinesProperty);
-        set => SetValue(LinesProperty, value);
-    }
-
-    public DiffLinesView() => InitializeComponent();
-}
diff --git a/src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml b/src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml
index 5d66da0c..003cdaf2 100644
--- a/src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml
+++ b/src/ClaudeDo.Ui/Views/Modals/DiffViewerView.axaml
@@ -162,10 +162,24 @@
               </Grid>
             </DockPanel>
 
-            <!-- Planning mode: flat aggregate/combined diff -->
+            <!-- Planning mode: one editor per file so each gets its own grammar -->
             <Grid Background="{DynamicResource VoidBrush}" IsVisible="{Binding IsPlanning}">
-              <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
-                <ctl:DiffLinesView Lines="{Binding DiffLines}"/>
+              <ScrollViewer VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Disabled">
+                <ItemsControl ItemsSource="{Binding PlanningFiles}">
+                  <ItemsControl.ItemTemplate>
+                    <DataTemplate x:DataType="vm:DiffFileViewModel">
+                      <StackPanel Margin="0,0,0,12">
+                        <Border Classes="island-header">
+                          <TextBlock Classes="path-mono" Text="{Binding HeaderPath}"
+                                     TextTrimming="PrefixCharacterEllipsis"/>
+                        </Border>
+                        <ctl:DiffTextView File="{Binding}"
+                                          IsSplit="{Binding $parent[Window].((vm:DiffViewerViewModel)DataContext).IsSplitView}"
+                                          WrapLines="{Binding $parent[Window].((vm:DiffViewerViewModel)DataContext).WrapLines}"/>
+                      </StackPanel>
+                    </DataTemplate>
+                  </ItemsControl.ItemTemplate>
+                </ItemsControl>
               </ScrollViewer>
             </Grid>