🤖 perf: speed up workspace-open transcript paint

[ammar-agent]

Summary

Follow-up perf pass for workspace-open rendering that reduces critical-path work in the transcript UI and message utilities. This improves startup paint times for loaded workspaces, especially on long tool-heavy histories.

Background

PR #2397 added automated perf profiling for workspace-open flows. Those artifacts showed recurring renderer-side hot spots during initial transcript paint (message-row prop churn, per-row grouping scans, and unnecessary live subscriptions after completion).

Implementation

• ChatPane render-path reductions
  • Precompute bash_output grouping once per message snapshot (computeBashOutputGroupInfos) instead of per-row scans.
  • Pass taskReportLinking only to task / task_await rows.
  • Precompute stable userMessageNavigation objects by historyId so non-message state bumps stop invalidating row props.
• Workspace caught-up sequencing
  • Defer the caught-up usageStore.bump() to idle (requestIdleCallback with timeout fallback) so initial transcript paint is prioritized.
• Reasoning row rendering
  • Use a plain truncated summary line in collapsed headers.
  • Avoid rendering full collapsed markdown body unless expanded/streaming.
• Bash row subscriptions
  • Gate live-output / latest-streaming / foreground-id subscriptions to rows that still need live state (executing or completed-without-final-output).
• Transcript truncation policy
  • Reduce MAX_DISPLAYED_MESSAGES from 128 → 64.
  • Preserve user prompts + structural markers by default; allow older assistant/tool/reasoning rows to stay behind history-hidden until “Load all”.
• Desktop dist/e2e stability fix
  • Replace @/... alias imports in src/desktop/main.ts with relative imports so dist runs do not emit unresolved alias require() calls.

Validation

• bun test src/browser/utils/messages/messageUtils.test.ts src/browser/utils/messages/StreamingMessageAggregator.test.ts src/browser/stores/WorkspaceStore.test.ts
• make static-check
• Perf scenario (dist build path):
  • MUX_E2E_RUN_PERF=1 MUX_PROFILE_REACT=1 MUX_E2E_LOAD_DIST=1 xvfb-run -a bun x playwright test tests/e2e/scenarios/perf.workspaceOpen.spec.ts --project=electron --workers=1

Perf wall-time (isolated profile runs)

Profile	Before (ms)	After (ms)	Delta
small	211	202	-4.3%
medium	212	186	-12.3%
large	244	200	-18.0%
tool-heavy	252	185	-26.6%
reasoning-heavy	174	171	-1.7%

Risks

• Transcript visibility tradeoff (intentional): older assistant rows can now be omitted behind history-hidden by default; users can restore with “Load all”.
• Reasoning header formatting tradeoff (intentional): collapsed summary is plain text (no markdown formatting) to avoid markdown render cost in headers.
• Bash subscription gating: completed rows now skip live subscriptions unless needed; behavior still preserves executing and completed-without-output cases.

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Generated with mux • Model: openai:gpt-5.3-codex • Thinking: xhigh • Cost: $4.95

[chatgpt-codex-connector]

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[ammar-agent]

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Codex Review: Didn't find any major issues. 🚀

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