[Mirror] fix(ui): throttle SSE part deltas and prevent stale delta text duplication

[pascalandr]

Mirror of NeuralNomadsAI#536 for Greptile review.\n\nSource PR: https://github.com/NeuralNomadsAI/CodeNomad/pull/536\nSource head: 105838b\nSource base: 4a1d53b\n\nThis mirror uses a dedicated base branch matching the upstream PR base so the diff stays limited to the original PR changes.

[greptile-apps]

Greptile Summary

This PR throttles SSE message.part.delta events through a 50 ms accumulation buffer to reduce redundant re-renders, and adds two synchronous flush paths (clearPendingDeltasForPart, flushPendingDeltasForMessage) to prevent stale buffered deltas from duplicating content when a full message.part.updated or message.updated event arrives first.

• Introduces enqueueDelta/flushDeltas with a module-level Map + setTimeout to coalesce part-delta SSE events into single applyPartDeltaV2 calls every 50 ms.
• Adds eager-flush hooks in both message.part.updated and message.updated handlers so server-authoritative full-state updates always land on a clean buffer.
• Extends findPendingSyntheticMessageId to also match messages in "sent" status, closing a race where a synthetic placeholder persisted past the server-acknowledged-but-not-yet-replaced window.

Confidence Score: 5/5

Safe to merge; the buffer/flush logic is sound and the eager-flush paths correctly prevent stale deltas from duplicating content.

The three main code paths — timer flush, part-update discard, and message-update eager flush — all use consistent patterns. The findPendingSyntheticMessageId fix to include "sent" is small and targeted. No data loss or duplication scenarios are introduced by the new buffering mechanism.

packages/ui/src/stores/session-events.ts — now at 851 lines, above the project's ~800-line target, and flushPendingDeltasForMessage mutates the Map while iterating its keys (prior review thread).

Important Files Changed

Filename	Overview
packages/ui/src/stores/session-events.ts	Adds 50 ms delta-throttle buffer, two eager-flush helpers, and a "sent"-status fix for synthetic message matching; logic is sound but flushPendingDeltasForMessage mutates the Map while iterating its keys (already flagged in prior review), and the file now sits at 851 lines above the project's ~800-line target.

Sequence Diagram

sequenceDiagram
    participant SSE as SSE Stream
    participant H as handleMessagePartDelta
    participant Q as pendingDeltas Map
    participant T as Timer 50ms
    participant B as applyPartDeltaV2

    SSE->>H: message.part.delta
    H->>Q: accumulate delta by instanceId:messageId:partId:field
    Q-->>T: arm timer if not running

    alt message.part.updated arrives first
        SSE->>Q: clearPendingDeltasForPart
        Note over Q: stale deltas discarded, full server state applied
    else message.updated arrives first
        SSE->>Q: flushPendingDeltasForMessage
        Q->>B: applyPartDeltaV2 for each pending key
        Note over B: all content applied before message marked complete
    else timer fires normally
        T->>Q: flushDeltas
        Q->>B: applyPartDeltaV2 for each batched entry
        Note over Q: Map cleared via Array.from snapshot
    end

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