fix(ui): stop ClearCompletedAsync from mutating chain-display fields

ClassifyItems() layers ApplyChainGrouping on top of partitioning, which
writes ShowAsChainMember/ChainStep/ChainAfterLabel on every row.
ClearCompletedAsync only needed the completed-section count/rows, so it
got that mutation as an unwanted side effect of counting.

Split the pure overdue/open/completed partition into PartitionItems();
ClassifyItems() now composes it with the chain-grouping mutation for
Regroup's actual render path. ClearCompletedAsync uses PartitionItems
directly. Regroup's behavior is unchanged.

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

@@ -529,10 +529,11 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     }
 
     // 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
+    // orphans flagged so they render flat) and partitions it into Overdue/Open/Completed. Pure
-    // as Regroup renders it. Shared with callers that need "every completed row" independent of
+    // with respect to chain state (ShowAsChainMember/ChainStep/ChainAfterLabel) — callers that
-    // Rows/IsShowingCompleted (e.g. ClearCompletedAsync).
+    // only need section membership/counts (e.g. ClearCompletedAsync) should use this directly
-    private (List<TaskRowViewModel> Overdue, List<TaskRowViewModel> Open, List<TaskRowViewModel> Completed) ClassifyItems()
+    // rather than ClassifyItems, which additionally mutates every row via ApplyChainGrouping.
+    private (List<TaskRowViewModel> Overdue, List<TaskRowViewModel> Open, List<TaskRowViewModel> Completed) PartitionItems()
     {
         // Items is already ordered by SortOrder from the DB query.
         // Treat rows whose ParentTaskId is not in the current view as orphans -> top-level.
@@ -580,6 +581,17 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
                 open.Add(r);
         }
 
+        return (overdue, open, completed);
+    }
+
+    // ClassifyItems layers the chain-grouping mutation on top of PartitionItems, exactly as
+    // Regroup renders it — every row in the result has ShowAsChainMember/ChainStep/ChainAfterLabel
+    // written. Only call this from a path that actually renders the result; a caller that just
+    // needs section membership (e.g. a count) should call PartitionItems instead.
+    private (List<TaskRowViewModel> Overdue, List<TaskRowViewModel> Open, List<TaskRowViewModel> Completed) ClassifyItems()
+    {
+        var (overdue, open, completed) = PartitionItems();
+
         // Dependency chains are resolved and pulled together per section (not on the
         // pre-split `flat` list): a chain head might land in a different section than its
         // dependent (e.g. a Done head in Completed, an open dependent in Open) — in that case
@@ -965,7 +977,7 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     [RelayCommand]
     private async Task ClearCompletedAsync()
     {
-        var (_, _, completed) = ClassifyItems();
+        var (_, _, completed) = PartitionItems();
         if (completed.Count == 0) return;
 
         // Delete children before parents so the parent-child FK (Restrict) doesn't

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

@@ -0,0 +1,156 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Ui.ViewModels.Islands;
+using Microsoft.EntityFrameworkCore;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+
+namespace ClaudeDo.Ui.Tests.ViewModels;
+
+// ClassifyItems (used by Regroup) additionally mutates every row's chain-display fields via
+// ApplyChainGrouping. ClearCompletedAsync only needs section membership/counts, so it goes
+// through the pure PartitionItems instead — this must not write ShowAsChainMember/ChainStep/
+// ChainAfterLabel as a side effect of counting completed rows.
+public class TasksIslandClassifyItemsTests : IDisposable
+{
+    private readonly string _dbPath;
+
+    public TasksIslandClassifyItemsTests()
+    {
+        _dbPath = Path.Combine(Path.GetTempPath(), $"claudedo_ui_classify_{Guid.NewGuid():N}.db");
+        using var ctx = NewContext();
+        ctx.Database.EnsureCreated();
+    }
+
+    public void Dispose()
+    {
+        try { File.Delete(_dbPath); } catch { }
+        try { File.Delete(_dbPath + "-wal"); } catch { }
+        try { File.Delete(_dbPath + "-shm"); } catch { }
+    }
+
+    private ClaudeDoDbContext NewContext()
+    {
+        var opts = new DbContextOptionsBuilder<ClaudeDoDbContext>()
+            .UseSqlite($"Data Source={_dbPath}")
+            .Options;
+        return new ClaudeDoDbContext(opts);
+    }
+
+    private sealed class TestDbFactory : IDbContextFactory<ClaudeDoDbContext>
+    {
+        private readonly Func<ClaudeDoDbContext> _create;
+        public TestDbFactory(Func<ClaudeDoDbContext> create) => _create = create;
+        public ClaudeDoDbContext CreateDbContext() => _create();
+    }
+
+    private TasksIslandViewModel BuildViewModel()
+    {
+        var factory = new TestDbFactory(NewContext);
+        return new TasksIslandViewModel(factory, worker: null);
+    }
+
+    private static ListNavItemViewModel UserList(string listEntityId, string name) =>
+        new() { Id = $"user:{listEntityId}", Kind = ListKind.User, Name = name };
+
+    private static async Task LoadAndWaitAsync(TasksIslandViewModel vm, ListNavItemViewModel list, int expectedCount = 1)
+    {
+        vm.LoadForList(list);
+        var deadline = DateTime.UtcNow.AddSeconds(5);
+        while (DateTime.UtcNow < deadline)
+        {
+            await Task.Delay(25);
+            if (vm.Items.Count >= expectedCount) break;
+        }
+        await Task.Delay(50);
+    }
+
+    private async Task SeedAsync(
+        params (string Id, TaskStatus Status, string? DependsOnTaskId, 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
+            {
+                Number = TestTaskNumbers.Next(),
+                Id = t.Id,
+                ListId = "list1",
+                Title = t.Id,
+                CreatedAt = DateTime.UtcNow,
+                Status = t.Status,
+                DependsOnTaskId = t.DependsOnTaskId,
+                SortOrder = t.SortOrder,
+            });
+        }
+        await db.SaveChangesAsync();
+    }
+
+    [Fact]
+    public async Task ClearCompleted_counting_path_leaves_chain_fields_untouched()
+    {
+        await SeedAsync(
+            ("head", TaskStatus.Idle, null, 0),
+            ("dep", TaskStatus.Idle, "head", 1));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"), expectedCount: 2);
+
+        var dep = vm.Items.Single(r => r.Id == "dep");
+        // Sanity: the initial Regroup (during load) computed the chain correctly.
+        Assert.True(dep.ShowAsChainMember);
+        Assert.Equal(1, dep.ChainStep);
+
+        // Corrupt the row to a sentinel distinguishable from both the correct and default value.
+        dep.ShowAsChainMember = false;
+        dep.ChainStep = 99;
+        dep.ChainAfterLabel = "SENTINEL";
+
+        // Nothing is completed, so ClearCompletedAsync does nothing but count via PartitionItems.
+        await vm.ClearCompletedCommand.ExecuteAsync(null);
+
+        Assert.False(dep.ShowAsChainMember);
+        Assert.Equal(99, dep.ChainStep);
+        Assert.Equal("SENTINEL", dep.ChainAfterLabel);
+    }
+
+    [Fact]
+    public async Task Regroup_still_recomputes_chain_fields()
+    {
+        await SeedAsync(
+            ("head", TaskStatus.Idle, null, 0),
+            ("dep", TaskStatus.Idle, "head", 1));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"), expectedCount: 2);
+
+        var dep = vm.Items.Single(r => r.Id == "dep");
+        dep.ShowAsChainMember = false;
+        dep.ChainStep = 99;
+        dep.ChainAfterLabel = "SENTINEL";
+
+        vm.Regroup();
+
+        Assert.True(dep.ShowAsChainMember);
+        Assert.Equal(1, dep.ChainStep);
+        Assert.Null(dep.ChainAfterLabel);
+    }
+
+    [Fact]
+    public async Task ClearCompleted_still_deletes_completed_rows_and_reports_count()
+    {
+        await SeedAsync(
+            ("open1", TaskStatus.Idle, null, 0),
+            ("done1", TaskStatus.Done, null, 1),
+            ("done2", TaskStatus.Done, null, 2));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"), expectedCount: 3);
+        vm.IsShowingCompleted = true;
+
+        await vm.ClearCompletedCommand.ExecuteAsync(null);
+
+        Assert.DoesNotContain(vm.Items, r => r.Id is "done1" or "done2");
+        Assert.Contains(vm.Items, r => r.Id == "open1");
+    }
+}