feat: root-as-head stack rendering with tree expansion

[comp615]

This PR description was generated by Amp.

Problem

The stack toggle in the main thread list groups handoff-related threads but has three issues:

1. Unstable head: The most recently updated thread becomes the stack head, which shifts as new children are added or threads are resumed
2. Flat expansion: Expanding a stack shows all other members sorted by date in a flat list, losing the tree structure
3. Forks are invisible: Given root → [branch-a → leaf-a, branch-b], expansion shows a flat interleaved list with no indication of which branch a thread belongs to

Before (flat list)

leaf-b (Jan 5)          ← head (most recent, shifts unpredictably)
  leaf-a (Jan 4)        ← flat, no structure
  branch-b (Jan 3)
  branch-a (Jan 2)
  root (Jan 1)

After (tree)

root (Jan 1) [5 threads · active Jan 5]     ← stable identity, sorted by most recent activity
  ├── branch-b (Jan 3)
  ├── branch-a (Jan 2)
  │   └── leaf-a (Jan 4)
  └── leaf-b (Jan 5)

Screenshot (after): 9-thread stack expanded as an indented tree with depth-based connector lines:

Solution

Commits follow dependency order:

1. feat: root-as-head stack rendering with tree expansion — Core behavior change: root thread is now the stack head, lastActiveDate drives sort position, expanded stacks render as indented trees via StackTree.tsx, DFS-ordered descendants field replaces ancestors
2. refactor: extract buildHandoffGraph to shared/utils and remove deprecated ancestors alias — Deduplicates the handoff graph-building logic (threadMap + parentToChildren + childToParent) into buildHandoffGraph() in shared/utils.ts, used by both threadStacks.ts (client) and threadChain.ts (server). Removes the deprecated ancestors alias from ThreadStack.

Key changes

Data model (shared/types.ts):

• ThreadStack.ancestors → ThreadStack.descendants (tree-ordered via DFS)
• Added ThreadStack.lastActiveDate (most recent lastUpdatedDate across all members)

Shared utility (shared/utils.ts):

• New buildHandoffGraph(threads) → { threadMap, childToParent, parentToChildren } — extracted from duplicated code in threadStacks.ts and threadChain.ts

Stack building (src/utils/threadStacks.ts):

• Head selection: root (no parent in stack) instead of most-recently-updated
• descendants built via DFS from root's children, sorted by lastUpdatedDate desc at each level
• Entries re-sorted by lastActiveDate desc so active stacks float to the top
• New getLastActiveThread() helper for kanban column placement

Tree rendering (src/components/ThreadList/StackTree.tsx):

• New recursive component that walks topology.parentToChildren from root
• Each node renders as a ThreadRow with stackDepth prop for indentation
• Children sorted by lastUpdatedDate desc for deterministic ordering

ThreadRow (src/components/ThreadList/ThreadRow.tsx, types.ts):

• Added stackDepth prop for depth-based indentation (marginLeft: (depth-1) * 16px)
• Added displayLastUpdated prop so the head row shows lastActiveDate (most recent activity) rather than the root's own last update

Kanban + Detail Card views: Updated to use descendants, topology, and getLastActiveThread() for correct column placement and tree-structured expansion.

Server (server/lib/threadChain.ts): Now uses buildHandoffGraph() from shared utils instead of inline graph construction.

React Strict Mode Fix

During development, discovered a bug where mutable Set objects were passed as props and mutated during render in StackTree. React strict mode double-renders components, so the second render pass would see the already-mutated Set and skip all nodes. Fix: create new Set instances instead of mutating shared state.

Dependencies

Depends on PR #59 (ccroom/fork-topology-data-model) which introduced ThreadStackTopology and fork-aware chain traversal.

Sibling PR: PR #69 (ccroom/full-tree-chain-display) — full-tree rendering in the chain discovery panel.

Verification

• pnpm typecheck passes (0 errors)
• All 256 tests pass (pnpm test)
• pnpm lint clean

AI Tool Validation

Copy and paste this prompt into your AI tool (Amp, Cursor, etc.):

Checkout comp615/thread-manager-for-amp branch ccroom/stack-tree-rendering and verify:

1. In src/utils/threadStacks.ts, confirm buildThreadStacks() uses the root thread
   (member with no parent) as the stack head, not the most recently updated thread.

2. In src/utils/threadStacks.ts, confirm entries are re-sorted by lastActiveDate
   (max lastUpdatedDate across all stack members) so active stacks float to top.

3. In src/components/ThreadList/StackTree.tsx, confirm the recursive tree renderer
   walks topology.parentToChildren from rootId and passes stackDepth to ThreadRow
   for indentation.

4. In shared/utils.ts, confirm buildHandoffGraph() builds threadMap, childToParent,
   and parentToChildren from threads' handoffParentId field.

5. Run `pnpm test` and confirm all tests pass.

[jom-sq]

i technically put the newest one on top on purpose at the time but yeah im fine with this!

[comp615]

i technically put the newest one on top on purpose at the time but yeah im fine with this!

@jom-sq yeah this one I feel the most awkward / unsure about. I think it both makes sense to have the newest on top, but also to see the tree to follow the thread as it were...maybe we chat live on this I do not feel as strongly about this one since the view vs sort is quirky