feat(ui): compute dependency-chain step/grouping in TasksIslandViewModel

Slice 1 of dependency-chain display: TaskRowViewModel gains DependsOnTaskId
plus the extrinsic ShowAsChainMember/ChainStep/ChainAfterLabel contract for
Slice 2's rail/badge rendering. Regroup's ClassifyItems walks each row's
DependsOnTaskId chain per section, pulls dependents directly under their
head regardless of SortOrder, and falls back to a flat row + label when the
head isn't in the same section (mirrors the ParentInView precedent).
Planning children never join a chain group - parent indent wins - and only
ever carry the label.

No AXAML changes; that's Slice 2.

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

@@ -28,12 +28,19 @@ public sealed partial class TaskRowViewModel : ViewModelBase
     [ObservableProperty] private string? _parentTaskId;
     [ObservableProperty] private string? _createdBy;
     [ObservableProperty] private string? _blockedByTaskId;
+    [ObservableProperty] private string? _dependsOnTaskId;
     [ObservableProperty] private bool _isExpanded = true;
     [ObservableProperty] private bool _hasPlanningChildren;
     [ObservableProperty] private bool _hasQueuedSubtasks;
     [ObservableProperty] private bool _showListChip = true;
     [ObservableProperty] private bool _parentFinalized;
     [ObservableProperty] private bool _parentInView = true;
+    // The three below are extrinsic — computed and assigned by TasksIslandViewModel.Regroup
+    // (it alone has the cross-row graph needed to walk DependsOnTaskId chains), mirroring how
+    // ParentInView is assigned by the same pass rather than derived locally.
+    [ObservableProperty] private bool _showAsChainMember;
+    [ObservableProperty] private int? _chainStep;
+    [ObservableProperty] private string? _chainAfterLabel;
     [ObservableProperty] private int _roadblockCount;
     // Only meaningful when Status=Failed; stamped by TaskRunner.MarkFailed. "unknown" for a
     // Failed task that predates this field. Null on every other status.
@@ -370,6 +377,7 @@ public sealed partial class TaskRowViewModel : ViewModelBase
         ParentTaskId      = t.ParentTaskId;
         CreatedBy         = t.CreatedBy;
         BlockedByTaskId   = t.BlockedByTaskId;
+        DependsOnTaskId   = t.DependsOnTaskId;
         RoadblockCount    = t.RoadblockCount;
         FailureReason     = t.FailureReason;
         FailureTurnsUsed  = t.FailureTurnsUsed;

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

@@ -580,9 +580,124 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
                 open.Add(r);
         }
 
+        // 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
+        // the two are never rendered adjacent, so the dependent must fall back to a flat row
+        // with a label rather than an orphaned rail. Whether the head is resolvable at all
+        // still uses the whole-Items graph (rowsById), independent of section.
+        var rowsById = Items.ToDictionary(r => r.Id);
+        overdue   = ApplyChainGrouping(overdue, rowsById);
+        open      = ApplyChainGrouping(open, rowsById);
+        completed = ApplyChainGrouping(completed, rowsById);
+
         return (overdue, open, completed);
     }
 
+    // Cycles are rejected by TaskStateService.SetDependsOnAsync, so this should always
+    // terminate quickly — capped anyway against a corrupt/legacy row forming a loop.
+    private const int MaxChainWalkDepth = 64;
+
+    private readonly record struct ChainWalkResult(TaskRowViewModel Head, int Step);
+
+    // Walks DependsOnTaskId back to its root. Returns null when the chain can't be resolved —
+    // either the row has no predecessor at all, or a predecessor along the way isn't loaded
+    // into the current Items (e.g. filtered out of this list/view).
+    private static ChainWalkResult? WalkChainHead(TaskRowViewModel row, Dictionary<string, TaskRowViewModel> rowsById)
+    {
+        var current = row;
+        var step = 0;
+        var visited = new HashSet<string> { row.Id };
+        while (!string.IsNullOrEmpty(current.DependsOnTaskId))
+        {
+            if (!rowsById.TryGetValue(current.DependsOnTaskId, out var predecessor)) return null;
+            if (step >= MaxChainWalkDepth || !visited.Add(predecessor.Id)) return null;
+            current = predecessor;
+            step++;
+        }
+        return step == 0 ? null : new ChainWalkResult(current, step);
+    }
+
+    // Content only — "after " + localization is Slice 2's concern (the view), which is why this
+    // holds just the identifying fragment rather than an assembled sentence.
+    private static string? BuildAfterLabel(TaskRowViewModel row, Dictionary<string, TaskRowViewModel> rowsById)
+    {
+        if (string.IsNullOrEmpty(row.DependsOnTaskId)) return null;
+        if (!rowsById.TryGetValue(row.DependsOnTaskId, out var predecessor)) return null;
+        return predecessor.Number > 0 ? $"#{predecessor.Number}" : predecessor.Title;
+    }
+
+    // Assigns ShowAsChainMember/ChainStep/ChainAfterLabel for every row in `section`, then
+    // returns a re-ordered copy with chain dependents pulled directly under their head
+    // (ascending ChainStep), regardless of their SortOrder-derived position — mirroring how
+    // planning children already sit right after their parent regardless of Items order.
+    private static List<TaskRowViewModel> ApplyChainGrouping(
+        List<TaskRowViewModel> section, Dictionary<string, TaskRowViewModel> rowsById)
+    {
+        var sectionIds = section.Select(r => r.Id).ToHashSet();
+        var headIdOf = new Dictionary<TaskRowViewModel, string>();
+        var stepOf = new Dictionary<TaskRowViewModel, int>();
+
+        foreach (var r in section)
+        {
+            if (string.IsNullOrEmpty(r.DependsOnTaskId))
+            {
+                r.ShowAsChainMember = false;
+                r.ChainStep = null;
+                r.ChainAfterLabel = null;
+                continue;
+            }
+
+            // A planning child keeps its parent indent — chain membership only ever shows as a
+            // label for it, never as a second, nested rail (design: "parent wins").
+            if (r.IsChild)
+            {
+                r.ShowAsChainMember = false;
+                r.ChainStep = null;
+                r.ChainAfterLabel = BuildAfterLabel(r, rowsById);
+                continue;
+            }
+
+            var walk = WalkChainHead(r, rowsById);
+            if (walk is { } w && sectionIds.Contains(w.Head.Id))
+            {
+                r.ShowAsChainMember = true;
+                r.ChainStep = w.Step;
+                r.ChainAfterLabel = null;
+                headIdOf[r] = w.Head.Id;
+                stepOf[r] = w.Step;
+            }
+            else
+            {
+                r.ShowAsChainMember = false;
+                r.ChainStep = null;
+                r.ChainAfterLabel = BuildAfterLabel(r, rowsById);
+            }
+        }
+
+        if (headIdOf.Count == 0) return section;
+
+        var membersByHeadId = section
+            .Where(headIdOf.ContainsKey)
+            .GroupBy(r => headIdOf[r])
+            .ToDictionary(g => g.Key, g => g.OrderBy(r => stepOf[r]).ToList());
+
+        var ordered = new List<TaskRowViewModel>(section.Count);
+        var consumed = new HashSet<string>();
+        foreach (var r in section)
+        {
+            if (consumed.Contains(r.Id)) continue;
+            if (headIdOf.ContainsKey(r)) continue; // placed via its head below, in step order
+            ordered.Add(r);
+            consumed.Add(r.Id);
+            if (membersByHeadId.TryGetValue(r.Id, out var members))
+                foreach (var m in members)
+                    if (consumed.Add(m.Id))
+                        ordered.Add(m);
+        }
+        return ordered;
+    }
+
     private void UpdateSubtitle()
     {
         var now     = DateTime.Now;

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

@@ -0,0 +1,245 @@
+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;
+
+// DependsOnTaskId chain display — Slice 1 (VM + Regroup grouping only, no AXAML). Design:
+// docs/superpowers/specs/2026-08-11-dependency-chain-display-design.md
+public class TasksIslandChainTests : IDisposable
+{
+    private readonly string _dbPath;
+
+    public TasksIslandChainTests()
+    {
+        _dbPath = Path.Combine(Path.GetTempPath(), $"claudedo_ui_test_{Guid.NewGuid():N}.db");
+        using var ctx = NewContext();
+        ctx.Database.EnsureCreated();
+    }
+
+    public void Dispose()
+    {
+        try { File.Delete(_dbPath); } catch { }
+        try { File.Delete(_dbPath + "-wal"); } catch { }
+        try { File.Delete(_dbPath + "-shm"); } catch { }
+    }
+
+    private ClaudeDoDbContext NewContext()
+    {
+        var opts = new DbContextOptionsBuilder<ClaudeDoDbContext>()
+            .UseSqlite($"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)
+    {
+        vm.LoadForList(list);
+        var deadline = DateTime.UtcNow.AddSeconds(5);
+        while (DateTime.UtcNow < deadline)
+        {
+            await Task.Delay(25);
+            if (vm.Items.Count > 0) break;
+        }
+        await Task.Delay(50);
+    }
+
+    private async Task<Dictionary<string, int>> SeedAsync(
+        params (string Id, TaskStatus Status, string? DependsOnTaskId, string? ParentTaskId, int SortOrder)[] tasks)
+    {
+        var numbers = new Dictionary<string, int>();
+        await using var db = NewContext();
+        db.Lists.Add(new ListEntity { Id = "list1", Name = "Default", CreatedAt = DateTime.UtcNow });
+        foreach (var t in tasks)
+        {
+            var number = TestTaskNumbers.Next();
+            numbers[t.Id] = number;
+            db.Tasks.Add(new TaskEntity
+            {
+                Number = number,
+                Id = t.Id,
+                ListId = "list1",
+                Title = t.Id,
+                CreatedAt = DateTime.UtcNow,
+                Status = t.Status,
+                DependsOnTaskId = t.DependsOnTaskId,
+                ParentTaskId = t.ParentTaskId,
+                SortOrder = t.SortOrder,
+            });
+        }
+        await db.SaveChangesAsync();
+        return numbers;
+    }
+
+    [Fact]
+    public async Task LinearChain_StepsIncreaseWithHopDistance()
+    {
+        await SeedAsync(
+            ("head", TaskStatus.Idle, null, null, 0),
+            ("s1", TaskStatus.Idle, "head", null, 1),
+            ("s2", TaskStatus.Idle, "s1", null, 2),
+            ("s3", TaskStatus.Idle, "s2", null, 3));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        var head = vm.Items.First(r => r.Id == "head");
+        var s1 = vm.Items.First(r => r.Id == "s1");
+        var s2 = vm.Items.First(r => r.Id == "s2");
+        var s3 = vm.Items.First(r => r.Id == "s3");
+
+        Assert.False(head.ShowAsChainMember);
+        Assert.Null(head.ChainStep);
+
+        Assert.True(s1.ShowAsChainMember);
+        Assert.Equal(1, s1.ChainStep);
+        Assert.True(s2.ShowAsChainMember);
+        Assert.Equal(2, s2.ChainStep);
+        Assert.True(s3.ShowAsChainMember);
+        Assert.Equal(3, s3.ChainStep);
+    }
+
+    [Fact]
+    public async Task TwoTasksSamePredecessor_ShareTheSameStep()
+    {
+        await SeedAsync(
+            ("head", TaskStatus.Idle, null, null, 0),
+            ("s1", TaskStatus.Idle, "head", null, 1),
+            ("s2a", TaskStatus.Idle, "s1", null, 2),
+            ("s2b", TaskStatus.Idle, "s1", null, 3));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        var s2a = vm.Items.First(r => r.Id == "s2a");
+        var s2b = vm.Items.First(r => r.Id == "s2b");
+
+        Assert.True(s2a.ShowAsChainMember);
+        Assert.True(s2b.ShowAsChainMember);
+        Assert.Equal(2, s2a.ChainStep);
+        Assert.Equal(2, s2b.ChainStep);
+    }
+
+    [Fact]
+    public async Task HeadInDifferentSection_DependentRendersFlatWithLabel()
+    {
+        // Same list, but "head" is Done (-> Completed section) while its dependent stays open.
+        // The two are never adjacent, so the dependent must not claim group membership.
+        var numbers = await SeedAsync(
+            ("head", TaskStatus.Done, null, null, 0),
+            ("dependent", TaskStatus.Idle, "head", null, 1));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        var dependent = vm.Items.First(r => r.Id == "dependent");
+
+        Assert.False(dependent.ShowAsChainMember);
+        Assert.Null(dependent.ChainStep);
+        Assert.NotNull(dependent.ChainAfterLabel);
+        Assert.Contains(numbers["head"].ToString(), dependent.ChainAfterLabel);
+    }
+
+    [Fact]
+    public async Task PlanningChildWithDependsOnTaskId_NeverBecomesChainMember()
+    {
+        await SeedAsync(
+            ("predecessor", TaskStatus.Idle, null, null, 0),
+            ("parent", TaskStatus.Idle, null, null, 1),
+            ("child", TaskStatus.Idle, "predecessor", "parent", 2));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        // Planning parents with children collapse by default — expand so the child is part of
+        // the flat list Regroup actually classifies (and thus reaches chain-grouping at all).
+        vm.ToggleExpandCommand.Execute(vm.Items.First(r => r.Id == "parent"));
+
+        var child = vm.Items.First(r => r.Id == "child");
+
+        Assert.True(child.IsChild);
+        Assert.False(child.ShowAsChainMember);
+        Assert.Null(child.ChainStep);
+        Assert.NotNull(child.ChainAfterLabel);
+    }
+
+    [Fact]
+    public async Task ChainGroup_IsPulledTogether_RegardlessOfSortOrder()
+    {
+        // "head" sorts after its dependents by SortOrder — grouping must still put the
+        // dependents directly after the head in Rows.
+        await SeedAsync(
+            ("s1", TaskStatus.Idle, "head", null, 0),
+            ("unrelated", TaskStatus.Idle, null, null, 1),
+            ("head", TaskStatus.Idle, null, null, 2));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        var rows = vm.Rows.OfType<TaskRowViewModel>().Select(r => r.Id).ToList();
+        var headIdx = rows.IndexOf("head");
+        var s1Idx = rows.IndexOf("s1");
+
+        Assert.Equal(headIdx + 1, s1Idx);
+    }
+
+    [Fact]
+    public async Task TaskWithoutDependencyOrDependents_IsUnaffected()
+    {
+        await SeedAsync(("lonely", TaskStatus.Idle, null, null, 0));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        var lonely = vm.Items.First(r => r.Id == "lonely");
+
+        Assert.False(lonely.ShowAsChainMember);
+        Assert.Null(lonely.ChainStep);
+        Assert.Null(lonely.ChainAfterLabel);
+    }
+
+    [Fact]
+    public async Task ChainProperties_RecomputeAfterTaskUpdated()
+    {
+        await SeedAsync(
+            ("head", TaskStatus.Idle, null, null, 0),
+            ("later", TaskStatus.Idle, null, null, 1));
+
+        var vm = BuildViewModel();
+        await LoadAndWaitAsync(vm, UserList("list1", "Default"));
+
+        var later = vm.Items.First(r => r.Id == "later");
+        Assert.False(later.ShowAsChainMember);
+
+        await using (var db = NewContext())
+        {
+            var entity = await db.Tasks.FirstAsync(t => t.Id == "later");
+            entity.DependsOnTaskId = "head";
+            await db.SaveChangesAsync();
+        }
+
+        await vm.RefreshTaskFromWorkerAsync("later");
+
+        var updated = vm.Items.First(r => r.Id == "later");
+        Assert.True(updated.ShowAsChainMember);
+        Assert.Equal(1, updated.ChainStep);
+    }
+}