perf(web): collect active tool calls without array shifts

[saphid]

Problem

deriveMessagesTimelineRows collects a running turn's contiguous tool and reasoning entries by scanning backward and calling activeToolEntries.unshift(entry) for each one. Every unshift moves the whole array, so a group of N entries costs N(N−1)/2 moves, and the projection is rebuilt on every streamed tool update, even when the group is collapsed. The initial 75-row snapshot does not bound this, because live updates keep accumulating on the client.

Change

Push during the backward scan, then reverse once. Same output, linear work.

Scope and approval

A two-line performance fix, so it uses the no-prior-issue exception.

Verification

• Benchmark of the real projector (3 warmups, median of 7) (base, fixed). The complete serialized output is identical to main in all six cases:

  Entries	Collapsed, main → PR	Expanded, main → PR
  1,000	1.69 → 1.51 ms	2.09 → 2.02 ms
  16,000	36.9 → 23.8 ms	35.7 → 20.3 ms
  32,000	101.4 → 37.8 ms	104.6 → 40.7 ms

• New MessagesTimeline.active-tools.test.ts locks the behavior over 1,000 entries: chronological grouped entries, the latest call's label, the latest thought, and group anchoring when collapsed or expanded. 162 passed with the existing logic tests (log), rerun after rebasing on current main.

• vp lint and apps/web typecheck: clean.

• Independent review by Claude Opus 5.5 (high) through T3 delegated tasks, two rounds: no defects.

Found by a GPT-6.1 Sol (high) audit and implemented by GPT-6.1 Sol (medium) in T3 Code. Reviewed by Claude Opus 5.5. Coordinated by Claude Opus 5.5 in Claude Code via T3 Code. Agent-authored, sponsored by Alex Southwell.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between ffb4af8 and 5b83c15.

📒 Files selected for processing (2)

• apps/web/src/components/chat/MessagesTimeline.active-tools.test.ts
• apps/web/src/components/chat/MessagesTimeline.logic.ts

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

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[macroscopeapp]

Approvability

Verdict: Approved at 5b83c15

Macroscope's review found this PR approvable — This is a narrowly scoped performance optimization that preserves the existing active-tool ordering and grouping while replacing quadratic array shifting with linear collection plus one reversal. The accompanying test covers the relevant long-group behavior, and no product defaults or static-analysis suppressions are changed.

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