fix(merge): harden 3-pane editor + document the new conflict resolver

Review follow-ups: coalesce gutter re-layout posts (avoid dispatcher flooding when
visual lines aren't ready), drop the zero-length deletable segment (undo hygiene),
and clear stale scroll-sync hooks on DataContext swap. Update Ui/CLAUDE.md to the
3-pane editor and log visual-verification items (incl. empty-side + alignment edges)
in docs/open.md.

src/ClaudeDo.Ui/Views/Conflicts/ConflictResolverView.axaml.cs

@@ -38,6 +38,7 @@ public partial class ConflictResolverView : Window
     private bool _rebuilding;
     private bool _applyingAccept;
     private bool _syncing;
+    private bool _gutterPending;
 
     public ConflictResolverView()
     {
@@ -53,6 +54,12 @@ public partial class ConflictResolverView : Window
             _vm.ActiveFileChanged -= Rebuild;
             _vm.CurrentChanged -= ScrollToCurrent;
         }
+        // The editors persist across a DataContext swap, so drop stale scroll-sync hooks first.
+        foreach (var sv in _scrollViewers)
+            if (sv is not null) sv.ScrollChanged -= OnPaneScroll;
+        _scrollViewers = Array.Empty<ScrollViewer?>();
+        _wired = false;
+
         _vm = DataContext as ConflictResolverViewModel;
         if (_vm is null) return;
 
@@ -150,7 +157,7 @@ public partial class ConflictResolverView : Window
             _wired = true;
             Dispatcher.UIThread.Post(HookScrollSync, DispatcherPriority.Loaded);
         }
-        Dispatcher.UIThread.Post(PositionGutters, DispatcherPriority.Loaded);
+        QueueGutters();
     }
 
     private static (string Text, List<(int Offset, int Length, MergeConflictBlock Block)> Spans) BuildSide(
@@ -185,7 +192,7 @@ public partial class ConflictResolverView : Window
         if (e.PropertyName is nameof(MergeConflictBlock.IsResolved) or nameof(MergeConflictBlock.Resolution))
         {
             InvalidateRenderers();
-            Dispatcher.UIThread.Post(PositionGutters, DispatcherPriority.Background);
+            QueueGutters();
         }
     }
 
@@ -202,7 +209,7 @@ public partial class ConflictResolverView : Window
                 break;
             }
         }
-        Dispatcher.UIThread.Post(PositionGutters, DispatcherPriority.Background);
+        QueueGutters();
     }
 
     // ── Accept a side into the result ────────────────────────────────────────
@@ -233,6 +240,14 @@ public partial class ConflictResolverView : Window
         RightGutter.Children.Clear();
     }
 
+    // Coalesce gutter re-layouts so repeated change/scroll events can't flood the dispatcher.
+    private void QueueGutters()
+    {
+        if (_gutterPending) return;
+        _gutterPending = true;
+        Dispatcher.UIThread.Post(() => { _gutterPending = false; PositionGutters(); }, DispatcherPriority.Background);
+    }
+
     private void PositionGutters()
     {
         ClearGutters();
@@ -240,7 +255,7 @@ public partial class ConflictResolverView : Window
         var tv = ResultEditor.TextArea.TextView;
         if (!tv.VisualLinesValid)
         {
-            Dispatcher.UIThread.Post(PositionGutters, DispatcherPriority.Background);
+            QueueGutters();
             return;
         }
 
@@ -314,7 +329,7 @@ public partial class ConflictResolverView : Window
         if (region.Block is null) return;
         var line = ResultEditor.Document.GetLineByOffset(region.Start.Offset).LineNumber;
         ResultEditor.ScrollToLine(line);
-        Dispatcher.UIThread.Post(PositionGutters, DispatcherPriority.Background);
+        QueueGutters();
     }
 
     private void InvalidateRenderers()
@@ -392,7 +407,6 @@ public partial class ConflictResolverView : Window
                 var s = Math.Max(segment.Offset, start);
                 var e = Math.Min(segment.EndOffset, end);
                 if (e > s) yield return new Seg(s, e - s);
-                else if (e == s && segment.Length == 0 && s >= start && s <= end) yield return new Seg(s, 0);
             }
         }
     }