refactor(ui): flatten TasksIsland's task list into one virtualized ListBox

Replace the three ItemsControl sections (Overdue/Open/Completed) with a single
flat Rows collection (HeaderRow | TaskRowViewModel) bound to a ListBox +
VirtualizingStackPanel, so the realized container count stays bounded instead
of growing with the list (measured: ~7-8 realized containers at any scroll
position with 1000 source rows). Group headers become regular row entries,
each section is simply omitted from Rows when empty, and the Completed
section's Clear-completed action moves onto the header row itself.

SectionFor/ReorderAsync/FindNextInSameSection now read a row's section off
the nearest preceding HeaderRow in Rows instead of three separate bound
collections, and ScrollSelectedIntoView goes through ListBox.ScrollIntoView so
it still works for rows outside the realized window. Drag/reorder behavior,
grouping/counts, and the Clear-completed button are unchanged.

Auto-scroll-while-dragging and the cross-section reorder bug are explicitly
out of scope (follow-up tasks).

src/ClaudeDo.Ui/Converters/NotHeaderRowConverter.cs

@@ -0,0 +1,18 @@
+using System.Globalization;
+using Avalonia.Data.Converters;
+using ClaudeDo.Ui.ViewModels.Islands;
+
+namespace ClaudeDo.Ui.Converters;
+
+// Drives ListBoxItem.Focusable for TasksIslandView's flat Rows list: a HeaderRow container must
+// not be a tab stop, a TaskRowViewModel container must behave like any other row.
+public class NotHeaderRowConverter : IValueConverter
+{
+    public static NotHeaderRowConverter Instance { get; } = new();
+
+    public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => value is not HeaderRow;
+
+    public object ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => throw new NotSupportedException();
+}

src/ClaudeDo.Ui/ViewModels/Islands/TasksIslandViewModel.cs

@@ -14,6 +14,19 @@ using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
 
 namespace ClaudeDo.Ui.ViewModels.Islands;
 
+/// <summary>A group header entry in <see cref="TasksIslandViewModel.Rows"/> — the flat list's
+/// other union member alongside <see cref="TaskRowViewModel"/>. Instances are persistent (one per
+/// section, mutated in place by <see cref="TasksIslandViewModel.Regroup"/>) so an unchanged header
+/// keeps its identity across a reconcile.</summary>
+public sealed partial class HeaderRow : ViewModelBase
+{
+    [ObservableProperty] private string _label = "";
+    [ObservableProperty] private int _count;
+    public bool IsOverdue { get; init; }
+    public IRelayCommand? ActionCommand { get; init; }
+    public bool HasAction => ActionCommand is not null;
+}
+
 public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
 {
     private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
@@ -57,9 +70,15 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     }
 
     public ObservableCollection<TaskRowViewModel> Items { get; } = new();
-    public ObservableCollection<TaskRowViewModel> OverdueItems { get; } = new();
+
-    public ObservableCollection<TaskRowViewModel> OpenItems { get; } = new();
+    /// <summary>Flat, virtualization-friendly view of <see cref="Items"/>: group headers
-    public ObservableCollection<TaskRowViewModel> CompletedItems { get; } = new();
+    /// (<see cref="HeaderRow"/>) interleaved with visible <see cref="TaskRowViewModel"/> rows, in
+    /// display order. Reconciled granularly by <see cref="Regroup"/> — never Clear+Add.</summary>
+    public ObservableCollection<object> Rows { get; } = new();
+
+    private readonly HeaderRow _overdueHeaderRow = new() { IsOverdue = true };
+    private readonly HeaderRow _openHeaderRow = new();
+    private readonly HeaderRow _completedHeaderRow;
 
     [ObservableProperty] private string _newTaskTitle = "";
     [ObservableProperty] private TaskRowViewModel? _selectedTask;
@@ -115,6 +134,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     {
         _dbFactory = dbFactory;
         _worker = worker;
+        _completedHeaderRow = new() { ActionCommand = ClearCompletedCommand };
         CompletedHeader = Loc.T("vm.tasksIsland.completedHeader");
         if (_worker is not null)
         {
@@ -138,7 +158,6 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     {
         CompletedHeader = Loc.T("vm.tasksIsland.completedHeader");
         foreach (var row in Items) row.RefreshLocalized();
-        foreach (var row in CompletedItems) row.RefreshLocalized();
     }
 
     private async void OnWorkerListUpdated(string listId)
@@ -309,9 +328,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
             _currentList.PropertyChanged += OnCurrentListPropertyChanged;
 
         Items.Clear();
-        OverdueItems.Clear();
+        Rows.Clear();
-        OpenItems.Clear();
-        CompletedItems.Clear();
         HasOverdue    = false;
         HasOpen       = false;
         HasCompleted  = false;
@@ -459,7 +476,54 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
                 r.IsExpanded = saved;
         }
 
-        // Build hierarchy-aware flat list: top-level rows interleaved with visible children.
+        var (overdue, open, completed) = ClassifyItems();
+
+        HasOverdue     = overdue.Count > 0;
+        HasOpen        = open.Count > 0;
+        HasCompleted   = completed.Count > 0;
+        ShowOpenLabel  = HasOpen && HasOverdue;
+        CompletedHeader = Loc.T("vm.tasksIsland.completedHeaderCount", completed.Count);
+
+        // Build the flat Rows target: header + rows per section, omitting a section entirely
+        // when it has nothing to show (no header, no rows) instead of hiding it via IsVisible.
+        var target = new List<object>();
+        if (HasOverdue)
+        {
+            _overdueHeaderRow.Label = Loc.T("tasks.overdue");
+            _overdueHeaderRow.Count = overdue.Count;
+            target.Add(_overdueHeaderRow);
+            target.AddRange(overdue);
+        }
+        if (HasOpen)
+        {
+            if (ShowOpenLabel)
+            {
+                _openHeaderRow.Label = Loc.T("tasks.tasks");
+                _openHeaderRow.Count = open.Count;
+                target.Add(_openHeaderRow);
+            }
+            target.AddRange(open);
+        }
+        if (HasCompleted && IsShowingCompleted)
+        {
+            _completedHeaderRow.Label = CompletedHeader;
+            _completedHeaderRow.Count = completed.Count;
+            target.Add(_completedHeaderRow);
+            target.AddRange(completed);
+        }
+
+        // Reconcile Rows in place (granular Insert/Move/Remove) rather than Clear+Add, so toggling
+        // a parent only touches its own child rows — the ListBox keeps every unchanged container
+        // instead of tearing the whole list down on a Reset.
+        SyncCollection(Rows, target);
+    }
+
+    // Builds the hierarchy-aware flat ordering (top-level rows interleaved with visible children,
+    // orphans flagged so they render flat) and classifies it into Overdue/Open/Completed, exactly
+    // as Regroup renders it. Shared with callers that need "every completed row" independent of
+    // Rows/IsShowingCompleted (e.g. ClearCompletedAsync).
+    private (List<TaskRowViewModel> Overdue, List<TaskRowViewModel> Open, List<TaskRowViewModel> Completed) ClassifyItems()
+    {
         // Items is already ordered by SortOrder from the DB query.
         // Treat rows whose ParentTaskId is not in the current view as orphans -> top-level.
         var visibleIds = Items.Select(r => r.Id).ToHashSet();
@@ -479,7 +543,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
             if (!emitted.Add(parent.Id)) continue;
             flat.Add(parent);
             // Also expand for Done parents so their (Done) children reach the classification
-            // loop and land in CompletedItems alongside the parent.
+            // loop and land alongside the parent in Completed.
             if ((parent.IsPlanningParent || parent.Done) && parent.IsExpanded)
             {
                 var children = Items.Where(r => r.ParentTaskId == parent.Id);
@@ -506,18 +570,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
                 open.Add(r);
         }
 
-        // Reconcile the bound collections in place (granular Insert/Move/Remove) rather than
+        return (overdue, open, completed);
-        // Clear+Add, so toggling a parent only touches its own child rows — the ItemsControl
-        // keeps every unchanged container instead of tearing the whole list down on a Reset.
-        SyncCollection(OverdueItems, overdue);
-        SyncCollection(OpenItems, open);
-        SyncCollection(CompletedItems, completed);
-
-        HasOverdue     = OverdueItems.Count > 0;
-        HasOpen        = OpenItems.Count > 0;
-        HasCompleted   = CompletedItems.Count > 0;
-        ShowOpenLabel  = HasOpen && HasOverdue;
-        CompletedHeader = Loc.T("vm.tasksIsland.completedHeaderCount", CompletedItems.Count);
     }
 
     private void UpdateSubtitle()
@@ -615,13 +668,13 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         // Master Items: single Move event (no Reset) so ItemsControls animate, not rebuild.
         MoveWithinCollection(Items, source, target, placeBelow);
 
-        // Apply the same move in whichever section the row lives in.
+        // Apply the same move in Rows, but only when both rows live in the same section.
         // Reorder never changes which section (Open/Overdue/Completed) a row belongs to —
         // that's determined by Done flag and ScheduledFor date, not drag-drop.
         var sourceSection = SectionFor(source);
         var targetSection = SectionFor(target);
         if (sourceSection is not null && ReferenceEquals(sourceSection, targetSection))
-            MoveWithinCollection(sourceSection, source, target, placeBelow);
+            MoveWithinCollection<object>(Rows, source, target, placeBelow);
 
         var listId = _currentList.Id["user:".Length..];
         var orderedIds = Items.Select(i => i.Id).ToList();
@@ -639,10 +692,10 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         await db.SaveChangesAsync();
     }
 
-    private static void MoveWithinCollection(
+    private static void MoveWithinCollection<T>(
-        System.Collections.ObjectModel.ObservableCollection<TaskRowViewModel> coll,
+        System.Collections.ObjectModel.ObservableCollection<T> coll,
-        TaskRowViewModel source,
+        T source,
-        TaskRowViewModel target,
+        T target,
         bool placeBelow)
     {
         var srcIdx = coll.IndexOf(source);
@@ -660,11 +713,11 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
 
     // Reconcile a bound collection toward a target order using granular Remove/Move/Insert,
     // so unchanged rows keep their containers (no Reset-driven full re-render).
-    private static void SyncCollection(
+    private static void SyncCollection<T>(
-        System.Collections.ObjectModel.ObservableCollection<TaskRowViewModel> dst,
+        System.Collections.ObjectModel.ObservableCollection<T> dst,
-        List<TaskRowViewModel> target)
+        List<T> target)
     {
-        var keep = new HashSet<TaskRowViewModel>(target);
+        var keep = new HashSet<T>(target);
         for (int i = dst.Count - 1; i >= 0; i--)
             if (!keep.Contains(dst[i]))
                 dst.RemoveAt(i);
@@ -680,12 +733,21 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         }
     }
 
-    private System.Collections.ObjectModel.ObservableCollection<TaskRowViewModel>? SectionFor(TaskRowViewModel row)
+    // Marker returned for a row that sits in the flat Open section before any header — Open is the
+    // only section that can render without a preceding HeaderRow (no Overdue and ShowOpenLabel
+    // false). Distinct from null, which means "row not found in Rows at all".
+    private static readonly object UnlabeledSection = new();
+
+    // A row's section is read off the nearest preceding HeaderRow in Rows rather than
+    // re-classified from Done/ScheduledFor, so it always matches what Regroup actually rendered
+    // (including planning-parent edge cases the classification alone can't reconstruct).
+    private object? SectionFor(TaskRowViewModel row)
     {
-        if (OverdueItems.Contains(row))   return OverdueItems;
+        var idx = Rows.IndexOf(row);
-        if (OpenItems.Contains(row))      return OpenItems;
+        if (idx < 0) return null;
-        if (CompletedItems.Contains(row)) return CompletedItems;
+        for (int i = idx - 1; i >= 0; i--)
-        return null;
+            if (Rows[i] is HeaderRow header) return header;
+        return UnlabeledSection;
     }
 
     /// <summary>
@@ -778,11 +840,12 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     [RelayCommand]
     private async Task ClearCompletedAsync()
     {
-        if (CompletedItems.Count == 0) return;
+        var (_, _, completed) = ClassifyItems();
+        if (completed.Count == 0) return;
 
         // Delete children before parents so the parent-child FK (Restrict) doesn't
         // block removing a completed planning parent together with its done children.
-        var toDelete = CompletedItems.OrderByDescending(r => r.IsChild).ToList();
+        var toDelete = completed.OrderByDescending(r => r.IsChild).ToList();
 
         if (ConfirmAsync is not null)
         {
@@ -1058,6 +1121,10 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     [RelayCommand]
     private void ToggleShowCompleted() => IsShowingCompleted = !IsShowingCompleted;
 
+    // The completed section is only emitted into Rows while the toggle is on — flipping it
+    // doesn't change grouping/order data, so a full reload isn't needed, just a re-emit.
+    partial void OnIsShowingCompletedChanged(bool value) => Regroup();
+
     public event EventHandler? OpenListSettingsRequested;
 
     [RelayCommand]

src/ClaudeDo.Ui/Views/Islands/TasksIslandView.axaml

@@ -2,7 +2,7 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:vm="using:ClaudeDo.Ui.ViewModels.Islands"
              xmlns:islands="using:ClaudeDo.Ui.Views.Islands"
-             xmlns:converters="using:Avalonia.Data.Converters"
+             xmlns:uiconverters="using:ClaudeDo.Ui.Converters"
              xmlns:loc="using:ClaudeDo.Ui.Localization"
              x:Class="ClaudeDo.Ui.Views.Islands.TasksIslandView"
              x:DataType="vm:TasksIslandViewModel">
@@ -89,82 +89,64 @@
             Command="{Binding OpenNotesCommand}"
             Content="{loc:Tr tasks.notesPinnedRow}"/>
 
-    <!-- Task list -->
+    <!-- Task list: one flat, virtualized ListBox. Rows is HeaderRow | TaskRowViewModel — group
-    <ScrollViewer>
+         headers are regular entries, not a separate ItemsControl per section, so a
-      <StackPanel Margin="10,4">
+         VirtualizingStackPanel actually bounds the realized container count. -->
-
+    <ListBox x:Name="RowsListBox"
-        <!-- OVERDUE -->
+             ItemsSource="{Binding Rows}"
-        <StackPanel IsVisible="{Binding HasOverdue}">
+             Background="Transparent"
-          <TextBlock Classes="eyebrow section-label overdue"
+             BorderThickness="0"
-                     Text="{loc:Tr tasks.overdue}" Margin="14,14,14,6"/>
+             Padding="10,4">
-          <ItemsControl ItemsSource="{Binding OverdueItems}">
+      <ListBox.ItemsPanel>
-            <ItemsControl.ItemTemplate>
+        <ItemsPanelTemplate>
-              <DataTemplate DataType="vm:TaskRowViewModel">
+          <VirtualizingStackPanel/>
-                <Button Classes="flat" HorizontalAlignment="Stretch"
+        </ItemsPanelTemplate>
-                        HorizontalContentAlignment="Stretch"
+      </ListBox.ItemsPanel>
-                        Command="{Binding $parent[ItemsControl].((vm:TasksIslandViewModel)DataContext).SelectCommand}"
+      <ListBox.Styles>
-                        CommandParameter="{Binding}">
+        <!-- SelectedItem is never bound — selection is handled entirely by the inner row Button's
-                  <islands:TaskRowView/>
+             SelectCommand, mirroring the pre-virtualization ItemsControl. Avalonia's ListBox has
-                </Button>
+             no "no selection" mode, so neutralize the app-wide selected/pointerover
-              </DataTemplate>
+             ContentPresenter overlay (App.axaml) instead of letting it double up with
-            </ItemsControl.ItemTemplate>
+             TaskRowView's own hover/selected style. -->
-          </ItemsControl>
+        <Style Selector="ListBoxItem">
-        </StackPanel>
+          <Setter Property="Padding" Value="0"/>
-
+          <Setter Property="MinHeight" Value="0"/>
-        <!-- TASKS -->
+          <Setter Property="CornerRadius" Value="0"/>
-        <StackPanel IsVisible="{Binding HasOpen}">
+          <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
-          <TextBlock Classes="eyebrow section-label"
+          <Setter Property="Focusable" Value="{Binding Converter={x:Static uiconverters:NotHeaderRowConverter.Instance}}"/>
-                     Text="{loc:Tr tasks.tasks}" Margin="14,14,14,6"
+        </Style>
-                     IsVisible="{Binding ShowOpenLabel}"/>
+        <Style Selector="ListBoxItem:pointerover /template/ ContentPresenter">
-          <ItemsControl ItemsSource="{Binding OpenItems}">
+          <Setter Property="Background" Value="Transparent"/>
-            <ItemsControl.ItemTemplate>
+        </Style>
-              <DataTemplate DataType="vm:TaskRowViewModel">
+        <Style Selector="ListBoxItem:selected /template/ ContentPresenter">
-                <Button Classes="flat" HorizontalAlignment="Stretch"
+          <Setter Property="Background" Value="Transparent"/>
-                        HorizontalContentAlignment="Stretch"
+        </Style>
-                        Command="{Binding $parent[ItemsControl].((vm:TasksIslandViewModel)DataContext).SelectCommand}"
+      </ListBox.Styles>
-                        CommandParameter="{Binding}">
+      <ListBox.DataTemplates>
-                  <islands:TaskRowView/>
+        <DataTemplate DataType="vm:HeaderRow">
-                </Button>
-              </DataTemplate>
-            </ItemsControl.ItemTemplate>
-          </ItemsControl>
-        </StackPanel>
-
-        <!-- COMPLETED -->
-        <StackPanel>
-          <StackPanel.IsVisible>
-            <MultiBinding Converter="{x:Static converters:BoolConverters.And}">
-              <Binding Path="HasCompleted"/>
-              <Binding Path="IsShowingCompleted"/>
-            </MultiBinding>
-          </StackPanel.IsVisible>
           <Grid ColumnDefinitions="*,Auto" Margin="14,14,14,6">
             <TextBlock Grid.Column="0" Classes="eyebrow section-label"
-                       Text="{Binding CompletedHeader}" VerticalAlignment="Center"/>
+                       Classes.overdue="{Binding IsOverdue}"
-            <Button Grid.Column="1" Classes="icon-btn"
+                       Text="{Binding Label}" VerticalAlignment="Center"/>
-                    Command="{Binding ClearCompletedCommand}"
+            <Button Grid.Column="1" Classes="icon-btn" IsVisible="{Binding HasAction}"
+                    Command="{Binding ActionCommand}"
                     ToolTip.Tip="{loc:Tr tasks.clearCompletedTip}"
                     VerticalAlignment="Center">
               <PathIcon Data="{StaticResource Icon.Trash}" Width="13" Height="13"
                         Foreground="{DynamicResource BloodBrush}"/>
             </Button>
           </Grid>
-          <ItemsControl ItemsSource="{Binding CompletedItems}">
+        </DataTemplate>
-            <ItemsControl.ItemTemplate>
+        <DataTemplate DataType="vm:TaskRowViewModel">
-              <DataTemplate DataType="vm:TaskRowViewModel">
+          <Button Classes="flat" HorizontalAlignment="Stretch"
-                <Button Classes="flat" HorizontalAlignment="Stretch"
+                  HorizontalContentAlignment="Stretch"
-                        HorizontalContentAlignment="Stretch"
+                  Command="{Binding $parent[ListBox].((vm:TasksIslandViewModel)DataContext).SelectCommand}"
-                        Command="{Binding $parent[ItemsControl].((vm:TasksIslandViewModel)DataContext).SelectCommand}"
+                  CommandParameter="{Binding}">
-                        CommandParameter="{Binding}">
+            <islands:TaskRowView/>
-                  <islands:TaskRowView/>
+          </Button>
-                </Button>
+        </DataTemplate>
-              </DataTemplate>
+      </ListBox.DataTemplates>
-            </ItemsControl.ItemTemplate>
+    </ListBox>
-          </ItemsControl>
-        </StackPanel>
-
-      </StackPanel>
-    </ScrollViewer>
 
   </DockPanel>
 </UserControl>

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

@@ -1,5 +1,4 @@
 using System;
-using System.Linq;
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;
@@ -62,19 +61,14 @@ public partial class TasksIslandView : UserControl
         };
     }
 
-    // Bring the selected row into view — the task list is a plain ItemsControl with no
+    // Bring the selected row into view — a programmatic select (e.g. Mission Control's "Open in
-    // built-in selection scrolling, so a programmatic select (e.g. Mission Control's
+    // app") can target a row that isn't currently realized. RowsListBox.ScrollIntoView is
-    // "Open in app") would otherwise highlight a row that stays off-screen.
+    // virtualization-aware (it realizes the container as needed); a visual-tree search for an
+    // existing Button, as this used to do, only finds rows already on screen.
     private void ScrollSelectedIntoView()
     {
         if (DataContext is not TasksIslandViewModel vm || vm.SelectedTask is not { } target) return;
-        Dispatcher.UIThread.Post(() =>
+        Dispatcher.UIThread.Post(() => RowsListBox.ScrollIntoView(target), DispatcherPriority.Background);
-        {
-            var match = this.GetVisualDescendants()
-                .OfType<Button>()
-                .FirstOrDefault(b => ReferenceEquals(b.DataContext, target));
-            match?.BringIntoView();
-        }, DispatcherPriority.Background);
     }
 
     private async System.Threading.Tasks.Task<bool> ShowConfirmAsync(string message)
@@ -363,13 +357,12 @@ public partial class TasksIslandView : UserControl
         return null;
     }
 
+    // The next row in Rows is only "in the same section" while it's a TaskRowViewModel —
+    // hitting a HeaderRow (or the end of the list) means row was the last one in its section.
     private static TaskRowViewModel? FindNextInSameSection(TasksIslandViewModel vm, TaskRowViewModel row)
     {
-        foreach (var section in new[] { vm.OverdueItems, vm.OpenItems, vm.CompletedItems })
+        var idx = vm.Rows.IndexOf(row);
-        {
+        if (idx < 0 || idx + 1 >= vm.Rows.Count) return null;
-            var idx = section.IndexOf(row);
+        return vm.Rows[idx + 1] as TaskRowViewModel;
-            if (idx >= 0) return idx + 1 < section.Count ? section[idx + 1] : null;
-        }
-        return null;
     }
 }

tests/ClaudeDo.Ui.Tests/ViewModels/TasksIslandRegroupTests.cs

@@ -1,3 +1,4 @@
+using System.Collections.Specialized;
 using ClaudeDo.Data;
 using ClaudeDo.Data.Models;
 using ClaudeDo.Ui.ViewModels.Islands;
@@ -84,6 +85,49 @@ public class TasksIslandRegroupTests : IDisposable
         await db.SaveChangesAsync();
     }
 
+    // Rows is the flat HeaderRow|TaskRowViewModel union — a row's section is read off the
+    // nearest preceding HeaderRow, mirroring TasksIslandViewModel.SectionFor. HasAction is
+    // unique to the Completed header (its Clear-completed button), so it disambiguates
+    // Completed from the (also non-overdue) Open header without depending on locale text.
+    private enum Section { None, Overdue, Open, Completed }
+
+    private static Section SectionOf(TasksIslandViewModel vm, TaskRowViewModel row)
+    {
+        var idx = vm.Rows.IndexOf(row);
+        if (idx < 0) return Section.None;
+        for (int i = idx - 1; i >= 0; i--)
+            if (vm.Rows[i] is HeaderRow h)
+                return h.IsOverdue ? Section.Overdue : h.HasAction ? Section.Completed : Section.Open;
+        return Section.Open;
+    }
+
+    private static bool IsInCompleted(TasksIslandViewModel vm, string taskId) =>
+        vm.Rows.OfType<TaskRowViewModel>().Any(r => r.Id == taskId && SectionOf(vm, r) == Section.Completed);
+
+    private static bool IsInOpen(TasksIslandViewModel vm, string taskId, bool requireChild = false) =>
+        vm.Rows.OfType<TaskRowViewModel>().Any(r =>
+            r.Id == taskId && (!requireChild || r.IsChild) && SectionOf(vm, r) == Section.Open);
+
+    private async Task SeedTasksAsync(params (string Id, TaskStatus Status, DateTime? ScheduledFor, int SortOrder)[] tasks)
+    {
+        await using var db = NewContext();
+        db.Lists.Add(new ListEntity { Id = "list1", Name = "Default", CreatedAt = DateTime.UtcNow });
+        foreach (var t in tasks)
+        {
+            db.Tasks.Add(new TaskEntity
+            {
+                Id = t.Id,
+                ListId = "list1",
+                Title = t.Id,
+                CreatedAt = DateTime.UtcNow,
+                Status = t.Status,
+                ScheduledFor = t.ScheduledFor,
+                SortOrder = t.SortOrder,
+            });
+        }
+        await db.SaveChangesAsync();
+    }
+
     private static ListNavItemViewModel VirtualList(string id, string name) =>
         new() { Id = id, Kind = ListKind.Virtual, Name = name };
 
@@ -171,10 +215,10 @@ public class TasksIslandRegroupTests : IDisposable
         // Parents with children collapse by default; expand to surface the nested child.
         vm.ToggleExpandCommand.Execute(vm.Items.First(r => r.Id == "p1"));
 
-        // Child with Done status under an open Planning parent should NOT go to CompletedItems
+        // Child with Done status under an open Planning parent should NOT land in the Completed section
-        Assert.DoesNotContain(vm.CompletedItems, r => r.Id == "c1");
+        Assert.False(IsInCompleted(vm, "c1"));
-        // Child should appear nested (IsChild == true) in OpenItems
+        // Child should appear nested (IsChild == true) in the Open section
-        Assert.Contains(vm.OpenItems, r => r.Id == "c1" && r.IsChild);
+        Assert.True(IsInOpen(vm, "c1", requireChild: true));
     }
 
     [Fact]
@@ -189,7 +233,120 @@ public class TasksIslandRegroupTests : IDisposable
         var vm = BuildViewModel();
         await LoadAndWaitAsync(vm, UserList("list1", "Default"));
 
-        Assert.Contains(vm.CompletedItems, r => r.Id == "p1");
+        Assert.True(IsInCompleted(vm, "p1"));
-        Assert.Contains(vm.CompletedItems, r => r.Id == "c1");
+        Assert.True(IsInCompleted(vm, "c1"));
+    }
+
+    [Fact]
+    public async Task Regroup_Rows_HeaderPositionsAndCountsMatchSections()
+    {
+        var yesterday = DateTime.Today.AddDays(-1);
+        await SeedTasksAsync(
+            ("overdue1", TaskStatus.Idle, yesterday, 0),
+            ("open1", TaskStatus.Idle, null, 1),
+            ("completed1", TaskStatus.Done, null, 2));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        Assert.Equal(6, vm.Rows.Count);
+
+        var overdueHeader = Assert.IsType<HeaderRow>(vm.Rows[0]);
+        Assert.True(overdueHeader.IsOverdue);
+        Assert.Equal(1, overdueHeader.Count);
+        Assert.Equal("overdue1", Assert.IsType<TaskRowViewModel>(vm.Rows[1]).Id);
+
+        // ShowOpenLabel is true here because Overdue is also present — the Open header IS emitted.
+        var openHeader = Assert.IsType<HeaderRow>(vm.Rows[2]);
+        Assert.False(openHeader.IsOverdue);
+        Assert.False(openHeader.HasAction);
+        Assert.Equal(1, openHeader.Count);
+        Assert.Equal("open1", Assert.IsType<TaskRowViewModel>(vm.Rows[3]).Id);
+
+        var completedHeader = Assert.IsType<HeaderRow>(vm.Rows[4]);
+        Assert.True(completedHeader.HasAction);
+        Assert.Equal(1, completedHeader.Count);
+        Assert.Equal("completed1", Assert.IsType<TaskRowViewModel>(vm.Rows[5]).Id);
+    }
+
+    [Fact]
+    public async Task Regroup_EmptySections_EmitNoHeader()
+    {
+        // Open-only: no Overdue means ShowOpenLabel is false too, so the Open header is never
+        // emitted either — Rows should hold exactly the one task row, nothing else.
+        await SeedTasksAsync(("open1", TaskStatus.Idle, null, 0));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        Assert.False(vm.HasOverdue);
+        Assert.False(vm.HasCompleted);
+        Assert.False(vm.ShowOpenLabel);
+        var row = Assert.Single(vm.Rows);
+        Assert.Equal("open1", Assert.IsType<TaskRowViewModel>(row).Id);
+    }
+
+    [Fact]
+    public async Task Regroup_CompletedSection_HiddenWhileIsShowingCompletedFalse()
+    {
+        await SeedTasksAsync(
+            ("open1", TaskStatus.Idle, null, 0),
+            ("completed1", TaskStatus.Done, null, 1));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+        Assert.True(IsInCompleted(vm, "completed1"));
+
+        vm.IsShowingCompleted = false;
+
+        Assert.DoesNotContain(vm.Rows.OfType<HeaderRow>(), h => h.HasAction);
+        Assert.DoesNotContain(vm.Rows.OfType<TaskRowViewModel>(), r => r.Id == "completed1");
+        // HasCompleted reflects the underlying data, independent of the toggle.
+        Assert.True(vm.HasCompleted);
+
+        vm.IsShowingCompleted = true;
+        Assert.True(IsInCompleted(vm, "completed1"));
+    }
+
+    [Fact]
+    public async Task Rows_ParentExpandCollapse_ReconcilesGranularly_NoResetEvent()
+    {
+        await SeedPlanningWithChildAsync(
+            parentStatus: TaskStatus.Idle, parentPhase: PlanningPhase.Active,
+            childStatus: TaskStatus.Idle,
+            parentId: "p1",
+            childId: "c1");
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        var actions = new List<NotifyCollectionChangedAction>();
+        vm.Rows.CollectionChanged += (_, e) => actions.Add(e.Action);
+
+        vm.ToggleExpandCommand.Execute(vm.Items.First(r => r.Id == "p1"));
+        vm.ToggleExpandCommand.Execute(vm.Items.First(r => r.Id == "p1"));
+
+        Assert.NotEmpty(actions);
+        Assert.DoesNotContain(NotifyCollectionChangedAction.Reset, actions);
+    }
+
+    [Fact]
+    public async Task ReorderAsync_WithinOpenSection_MovesRowInItemsAndRows()
+    {
+        await SeedTasksAsync(
+            ("t1", TaskStatus.Idle, null, 0),
+            ("t2", TaskStatus.Idle, null, 1),
+            ("t3", TaskStatus.Idle, null, 2));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        var t1 = vm.Items.First(r => r.Id == "t1");
+        var t3 = vm.Items.First(r => r.Id == "t3");
+
+        await vm.ReorderAsync(t1, t3, placeBelow: true);
+
+        Assert.Equal(new[] { "t2", "t3", "t1" }, vm.Items.Select(r => r.Id));
+        Assert.Equal(new[] { "t2", "t3", "t1" }, vm.Rows.OfType<TaskRowViewModel>().Select(r => r.Id));
     }
 }

tests/ClaudeDo.Worker.Tests/UiVm/TasksIslandViewModelPlanningTests.cs

@@ -211,6 +211,19 @@ public class TasksIslandViewModelPlanningTests
         PlanningPhase phase = PlanningPhase.None)
         => new TaskRowViewModel { Id = id, Status = status, ParentTaskId = parentId, PlanningPhase = phase };
 
+    // vm.Rows is the flat HeaderRow|TaskRowViewModel union — a row is "in the Open section" when
+    // it's present in Rows and the nearest preceding HeaderRow (if any) isn't Overdue/Completed.
+    // HasAction uniquely identifies the Completed header (its Clear-completed button).
+    private static bool IsVisibleInOpen(TasksIslandViewModel vm, TaskRowViewModel row)
+    {
+        var idx = vm.Rows.IndexOf(row);
+        if (idx < 0) return false;
+        for (int i = idx - 1; i >= 0; i--)
+            if (vm.Rows[i] is HeaderRow h)
+                return !h.IsOverdue && !h.HasAction;
+        return true;
+    }
+
     [Fact]
     public void PlanningParentWithChildren_CollapsedByDefault_ToggleExpands()
     {
@@ -221,16 +234,16 @@ public class TasksIslandViewModelPlanningTests
         var (vm, _) = VmFactory.Create([parent, child1, child2]);
 
         // Collapsed by default — children hidden, parent still present
-        Assert.DoesNotContain(child1, vm.OpenItems);
+        Assert.False(IsVisibleInOpen(vm, child1));
-        Assert.DoesNotContain(child2, vm.OpenItems);
+        Assert.False(IsVisibleInOpen(vm, child2));
-        Assert.Contains(parent, vm.OpenItems);
+        Assert.True(IsVisibleInOpen(vm, parent));
 
         // Expand the parent
         vm.ToggleExpandCommand.Execute(parent);
 
         // Children now visible
-        Assert.Contains(child1, vm.OpenItems);
+        Assert.True(IsVisibleInOpen(vm, child1));
-        Assert.Contains(child2, vm.OpenItems);
+        Assert.True(IsVisibleInOpen(vm, child2));
     }
 
     [Fact]
@@ -268,15 +281,15 @@ public class TasksIslandViewModelPlanningTests
         var (vm, _) = VmFactory.Create([parent, child]);
 
         // Collapsed by default
-        Assert.DoesNotContain(child, vm.OpenItems);
+        Assert.False(IsVisibleInOpen(vm, child));
 
         // Expand
         vm.ToggleExpandCommand.Execute(parent);
-        Assert.Contains(child, vm.OpenItems);
+        Assert.True(IsVisibleInOpen(vm, child));
 
         // Collapse again
         vm.ToggleExpandCommand.Execute(parent);
-        Assert.DoesNotContain(child, vm.OpenItems);
+        Assert.False(IsVisibleInOpen(vm, child));
     }
 
     [Fact]
@@ -291,7 +304,7 @@ public class TasksIslandViewModelPlanningTests
         Assert.False(orphan.ParentInView);
         Assert.False(orphan.ShowAsChild);
         Assert.False(orphan.IsDraft);
-        Assert.Contains(orphan, vm.OpenItems);
+        Assert.True(IsVisibleInOpen(vm, orphan));
     }
 
     [Fact]