fix(web-ui): keep long sessions visible after send

[limityan]

Summary

• Keep the previous visible Flow Chat tail mounted during same-session initial-history transitions after sending from a historical session.
• Avoid treating layout/programmatic scrolls near a partial-history boundary as user full-history intent.
• Split follow-output ownership from sticky-latest pin permission so restored streaming sessions keep follow-output behavior while user-only in-flight turns do not get sticky-pinned before model output exists.
• Add generated long-session E2E coverage for the send-after-open blank/reload regression.

Root cause

This was not a single loading-state issue. In debug mode with long historical sessions, sending a new message can change the latest turn and initial-history render window while the same session is still moving between partial tail, context restore, and full projection states. The existing projection handoff protected session switches, but not this same-session initial-history transition, so the message DOM could briefly collapse to a blank/reloaded surface.

Two adjacent contributors made the blank easier to hit: passive scroll events from layout changes could schedule full-history projection as if the user had scrolled upward, and sticky-latest pinning was applied to a user-only processing turn before model output existed.

A follow-up adversarial review found that the first sticky-pin guard was too broad: it also made the container treat restored processing sessions as completed history and broke the existing follow-output handoff test. The fix now keeps two separate policies: follow-output owns active user turns, while sticky-latest pin starts only after model output exists.

Latest adversarial review

• Rebased onto gcwing/main at 0f0f70d3 and rechecked the full PR diff from that base.
• No additional source changes were required after the latest review.
• Performance risk review: the added static-history bottom guard is bounded to a short 2.5s window, cancels on real upward user intent, and does not request full-history projection from passive layout scrolls. Latest debug reports show no post-send full-history projection requests.

Validation

• pnpm --dir src/web-ui run test:run (221 files / 1249 tests)
• pnpm run type-check:web
• pnpm run lint:web
• pnpm run build:web
• git diff --check gcwing/main...HEAD
• Generated clean long-session fixture: 20 sessions, target session 60 turns, 1500 assistant chars, 6000 tool-result chars, 2 tool items.
• Debug E2E send-after-open, clean fixture, repeated 3x: keeps a generated long session visible after sending a new message
  • reports: long-session-send-after-open-2026-06-29T06-25-36-819Z.json, long-session-send-after-open-2026-06-29T06-28-44-604Z.json, long-session-send-after-open-2026-06-29T06-28-59-073Z.json
  • each run: post-user blank events 0, post-latest blank/loading events 0, loading / overlay / placeholder visual toggles 0 / 0 / 0, full-history projection after send 0
• Debug E2E warm-reopen, clean fixture: collects warm-reopen timing for a generated long session
  • report: long-session-warm-reopen-2026-06-29T06-30-21-051Z.json
  • latest content and model round visible: true / true
  • loading / overlay / placeholder visual toggles: 0 / 0 / 0

The local debug E2E environment has no primary model configured, so the test turn eventually fails server-side and is restored to the pending queue by existing error handling. The regression assertion covers the UI staying visible through send, failure, and restoration.

Fixes #1321