Summary

• keep header turn state tied to the real visible viewport while pending turn navigation retries in the background
• preserve static initial-history scroll ownership when users read older content, including bottom-guard, manual-scroll, and return-to-latest paths
• keep latest content reachable after static anchor-window jumps by preserving tail space and deferring latest scroll until the latest DOM is restored
• add release-compatible E2E coverage for long-session header turn navigation

Validation

• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/ModernFlowChatContainer.history-state.test.tsx src/flow_chat/components/modern/VirtualMessageList.layout.test.ts src/flow_chat/components/modern/VirtualMessageList.session-boundary.test.tsx
• pnpm run type-check:web
• pnpm run desktop:build:release-fast
• pnpm --dir tests/e2e exec wdio run ./config/wdio.conf.ts --spec ./specs/l1-chat-turn-navigation-release.spec.ts
• node tests/e2e/scripts/run-long-session-interaction-matrix.mjs --profile scroll
• final independent architecture/product subagent review: no remaining must-fix

Fixes #1281
Related to #1381