perf(threads): reuse derived state while loading activity

[extoci]

Summary

Long-running web threads repeatedly rebuilt work-log, timeline, and row projections when activity or messages changed. This change reuses immutable projection state across exact appends, keeps active timeline scan state with its owning timeline, and incrementally accumulates work-group presentation data.

The optimization is deliberately scoped to the web client. Mobile call sites are unchanged and can move to the shared incremental APIs in a follow-up PR.

Measured impact

Measured on a read-only snapshot of the current local state: 174 activity-bearing threads and 77,748 activities. Values are median projection times on the same machine; the average row is the real 441-activity thread nearest the corpus mean of 446.8 activities. The 2× and 10× cases duplicate the real maximum-thread activity shape with unique identities.

Scenario	Activities	Baseline	This branch	Improvement
Minimum activity thread	1	0.066 ms	0.066 ms	0.3% less time
Representative average thread	441	4.18 ms	1.46 ms	65.1% less time
Maximum activity thread	8,204	166.95 ms	40.86 ms	75.5% less time
2× maximum thread	16,408	360.28 ms	77.29 ms	78.5% less time
10× maximum thread	82,040	1,814.72 ms	387.45 ms	78.6% less time

For 100 live updates using the maximum-thread shape, the full projection fanout went from 23.83 s to 22.88 s: 4.0% less time. The cold-load improvement is larger because it removes repeated full sorting, folding, and presentation work; the remaining live cost includes other projections that this PR does not change.

Validation

• 220 focused tests passed across web and shared presentation logic.
• Web and client-runtime typechecks passed.
• Targeted lint and formatting passed.
• The benchmark database was opened read-only. No live userdata was modified.
• Mobile UI changes are intentionally not included.

Model: GPT-5.6
Harness: Codex

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Note

Medium Risk
Incremental projection logic is subtle (prefix checks, WeakMaps, lifecycle edge cases); bugs could show wrong tool groups, stale timeline rows, or incorrect changed-file chips without failing tests on typical threads.

Overview
Speeds up long web chat threads by reusing derived state when messages, plans, and activities only grow at the end, instead of rebuilding work-log folds, merged timeline rows, and message-timeline rows on every tick.

ChatView now keeps WorkLogEntriesProjection and TimelineEntriesProjection in refs and calls deriveWorkLogEntriesWithState / deriveTimelineEntriesWithState, which clone fold indexes only for the new activity suffix and merge timeline entries while preserving stable object identity where possible.

MessagesTimeline owns an activeTimelineScanRef (reset on thread change) so live “active tool” scanning and work-group presentation can append incrementally via WeakMap-anchored caches and ToolGroupSummaryAccumulator from client-runtime, with sliced snapshots for expanded groups so UI arrays stay immutable.

session-logic also tightens work-log metadata: changedFiles only for mutating tool types (not dynamic reads), faster activity ordering/pending-derivation paths, and small command/detail parsing optimizations.

Tests cover projection reuse, branch-safe snapshots, per-timeline scan isolation, and changed-file rules.

^{Reviewed by Cursor Bugbot for commit 71dbdae. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Reuse derived work-log and timeline state for incremental activity loading

• Adds stateful projection types (WorkLogEntriesProjection, TimelineEntriesProjection) that cache derived entries and fold state. When the prior source arrays are an exact immutable prefix, only the suffix is derived; otherwise the full path rebuilds state.
• Threads per-owner active timeline scan refs through deriveMessagesTimelineRows and the MessagesTimeline component so active-turn scanning reuses prior scan state across renders.
• Moves tool-group summaries to a ToolGroupSummaryAccumulator and switches activity ordering to a cached orderedActivities helper.
• Fixes changed-file extraction in toDerivedWorkLogEntry to retain paths from MCP tool results and mutating dynamic tools while suppressing read-tool paths; prefers item.command in extractToolCommand and unifies the command preview in extractToolDetail.
• Fixes deriveTurnFolds so unsettled-turn entries are skipped from the settled-turn fold map.
• Behavioral Change: toolGroupSummaryKind now records an entry's action in allActions before handling toolSource entries, so source-bearing entries participate in single-action vs mixed classification. Branch-point projections in deriveWorkLogEntriesWithState and deriveTimelineEntriesWithState must remain immutable after creation; mutating a returned projection's source arrays will break suffix reuse.

^{Macroscope summarized 71dbdae.}

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR replaces existing activity and timeline derivation with stateful incremental projections across multiple production modules, including lifecycle folding, grouping, ordering, and shared summaries. The cross-cutting new state management and behavior changes make it broader than a low-risk isolated performance tweak.

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