fix(desktop): role-aware tail-first stamping (re-land of #364)

[Kyzcreig]

What

Re-lands #364 (role-aware tail-first stamping) onto current main. The original branch fell 705 commits behind; its hook file conflicted with the livesync Phase-1 work landed since.

The live mechanism (CDP-attach, 2026-07-15): a stale optimistic assistant row — its completion frame severed by a backend restart — absorbed the fresh turn's user_id via the old top-down role-blind walk. The zombie then wore a committed id (invisible to the #361 sweep) and painted its stale text as a permanent duplicate.

Both LIVE Greptile findings adjudicated (one fixed-by-documentation, one fixed-by-hardening)

1. Silent user-id drop when no user-role stampable row exists — this is deliberate, now documented in-code and test-pinned: falling back to any-role would stamp the USER id onto an assistant row, re-creating the exact cross-role mis-stamp this PR exists to prevent. An unstamped-but-committed id is safe (the poll reconciles it with no optimistic twin to duplicate); a cross-role-stamped zombie is not.
2. 3+-id frames (message_ids array + scalar fields both populated) — now handled by the same assistant-preferring tail-first walk: no crash, extras left for the poll, each id/row claimed at most once. Test-pinned.

Verification

• RED-proven: swapping the old fork/main impl back in (keeping the new tests) fails 5/36 — the 3 zombie vectors + both edge-frame tests.
• Hook suite: 36/36 green.
• Full desktop suite delta vs clean fork/main baseline in an identical worktree: zero new failures (17 pre-existing reds on both trees — local-env; the 1 extra on my run passes 3/3 in isolation = flake).
• tsc --noEmit: identical 7 pre-existing errors on both trees.

#364 will be closed as superseded with credit (Co-authored-by).

[greptile-apps]

Greptile Summary

This PR re-lands the role-aware tail-first stamping algorithm (stampOptimisticTranscriptRows) that prevents stale assistant zombie rows from absorbing committed IDs during turn completion. Instead of the old linear role-blind walk, IDs are now claimed from the tail of the stampable list by matching role — user IDs go only to user rows, assistant IDs only to assistant rows — so zombies from a severed prior turn keep their optimistic IDs and remain sweepable.

• The core 2-id path correctly prevents cross-role mis-stamping and is well-tested with the zombie scenario described in the live incident (2026-07-15).
• The 3+-id extras loop (for frames where array and scalar fields are both populated) does not deduplicate committedIds before iterating; a duplicate ID from extractCommittedMessageIds can slip through and stamp a zombie with a committed ID that is already assigned to the user row, recreating the unsweepable-zombie duplicate-display bug in that specific combination.

Confidence Score: 4/5

The 2-id stamping path — the primary fix for the live zombie mis-stamp incident — is correct and well-tested. The 3+-id extras loop has a gap: when extractCommittedMessageIds returns duplicate IDs (array + scalar fields overlap) and a zombie row is unclaimed, the zombie can absorb a committed ID already assigned to the user row, recreating the unsweepable duplicate-display bug.

The core role-aware tail-first algorithm works correctly for the 2-id case covering the documented live incident. The 3+-id extras branch introduces a scenario where duplicate IDs from overlapping array and scalar fields can propagate a committed ID onto a zombie row — the exact failure mode the PR is hardening against. Deduplicating committedIds before branching would close the gap with minimal risk.

apps/desktop/src/app/chat/hooks/use-session-changes.ts — specifically the 3+-id extras loop at lines 428-432 and the absence of deduplication on committedIds before the length-based branch.

Important Files Changed

Filename	Overview
apps/desktop/src/app/chat/hooks/use-session-changes.ts	Replaces the linear role-blind stamp walk with a role-aware tail-first algorithm; the 3+-id extras loop can stamp a zombie with a duplicate committed ID when extractCommittedMessageIds returns overlapping array+scalar ids
apps/desktop/src/app/chat/hooks/use-session-changes.test.ts	Adds two new describe blocks (9 tests) covering the zombie mis-stamp and edge-frame scenarios; the 3+-id test uses distinct ids without a zombie row, leaving the duplicate-id + zombie combination untested

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[stampOptimisticTranscriptRows called] --> B{committedIds.length?}
    B -- 0 --> C[Return unchanged messages]
    B -- 1 --> D[Lone-id path: prefer assistant tail, else any]
    B -- 2 --> E[2-id positional path]
    B -- 3+ --> F[3+-id path]

    E --> E1["claim(ids[0], 'user')"]
    E1 --> E2["claim(ids[1], 'assistant')"]
    E2 --> G[Build final messages via assignment map]

    F --> F1["claim(ids[0], 'user')"]
    F1 --> F2["claim(ids[1], 'assistant')"]
    F2 --> F3["for each extra id: claim assistant else any"]
    F3 -- "duplicate id + unclaimed zombie" --> F4["Zombie stamped with duplicate committed id — unsweepable"]
    F3 -- "all stampable rows claimed" --> G

    D --> D1["claim(id, 'assistant')"]
    D1 -- fail --> D2["claim(id, null)"]
    D1 -- success --> G
    D2 --> G

    G --> H[Return messages + stampedIds]
    H --> I[Poll: dropZombieOptimisticRows sweeps remaining optimistic rows]

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
    A[stampOptimisticTranscriptRows called] --> B{committedIds.length?}
    B -- 0 --> C[Return unchanged messages]
    B -- 1 --> D[Lone-id path: prefer assistant tail, else any]
    B -- 2 --> E[2-id positional path]
    B -- 3+ --> F[3+-id path]

    E --> E1["claim(ids[0], 'user')"]
    E1 --> E2["claim(ids[1], 'assistant')"]
    E2 --> G[Build final messages via assignment map]

    F --> F1["claim(ids[0], 'user')"]
    F1 --> F2["claim(ids[1], 'assistant')"]
    F2 --> F3["for each extra id: claim assistant else any"]
    F3 -- "duplicate id + unclaimed zombie" --> F4["Zombie stamped with duplicate committed id — unsweepable"]
    F3 -- "all stampable rows claimed" --> G

    D --> D1["claim(id, 'assistant')"]
    D1 -- fail --> D2["claim(id, null)"]
    D1 -- success --> G
    D2 --> G

    G --> H[Return messages + stampedIds]
    H --> I[Poll: dropZombieOptimisticRows sweeps remaining optimistic rows]

Loading

_{Reviews (2): Last reviewed commit: "fix(desktop): 3+-id frames keep the posi..." | Re-trigger Greptile}