Summary

• Defer restored historical-session background command snapshots until the latest restored content is visible, so non-critical session metadata work does not compete with the first readable frame.
• Delay activity touch for metadata-only historical sessions until hydration completes and the target session actually switches, avoiding stale side work during rapid switching.
• Expand the initial render window only when the latest restored turn is user-only, preserving recent context without applying a global extra-turn DOM cost.
• Tighten the performance E2E workspace setup and add a clearer rapid-switch breakdown for target-click, latest-visible, latest-usable, and per-session trace phases.

Performance Data

Measured on release-fast builds, 5 runs each, same generated long-session fixture. Baseline is latest GCWing/BitFun main at bc3e5f33; this PR is 5fb8184.

Startup is included only as a guardrail. The measured +4.1 ms startup delta is within run-to-run noise and this PR should not be interpreted as a startup optimization.

Rapid-Switch Breakdown

Diagnostic sample from the latest PR run, measured from the first click in a rapid A -> B -> C session switch sequence:

Per-session trace sample from that run:

Risks and Guardrails

• Background command activity can appear slightly later on restored historical sessions, but live command events remain the source of truth and the snapshot still runs once latest content is visible.
• User-only latest turns render one extra previous turn during initial history paint; the extra DOM cost is conditional and the rapid-switch E2E path remains faster than baseline.
• Activity touch is no longer sent for a superseded metadata-only switch. This avoids stale recency updates during rapid switch, but failed or superseded opens also stop updating recency.
• Rapid-switch breakdown is test/report-only. It does not add production instrumentation or runtime logs; the console summary uses compact relative timings.
• The perf E2E helper opt-out only applies to this perf spec; default workspace tests still require nav label visibility.

Verification

• Rebased onto latest GCWing/BitFun main at bc3e5f33
• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/VirtualMessageList.layout.test.ts src/flow_chat/components/modern/ModernFlowChatContainer.history-state.test.tsx src/flow_chat/services/flow-chat-manager/SessionModule.test.ts (51 tests passed)
• pnpm run lint:web (0 errors; existing warning in untouched FlowChatManager.ts)
• pnpm run check:repo-hygiene
• git diff --check gcwing/main..HEAD
• pnpm run desktop:build:release-fast for baseline and PR builds
• pnpm --dir tests/e2e run test:perf x5 for latest main baseline and x5 for this PR