Fix FlowChat tail-follow transitions and delayed shrink jumps

[wsp1911]

Summary

Improves FlowChat viewport stability across pinned-turn handoff, smooth tail follow, animated card collapse, and delayed virtualizer measurements.

Fixes #

Type and Areas

Type:

Regression fix, performance/stability, test, docs

Areas:

Web UI / FlowChat

Motivation / Impact

FlowChat could fail to transition from a pinned user message into tail follow, visibly jump during smooth follow, or drop when Virtuoso committed a delayed item shrink after collapse protection had been released.

This change:

• Makes pinned-to-tail handoff depend on real natural-tail geometry.
• Uses one RAF writer for smooth streaming follow.
• Detects strict browser shrink-clamp signatures and restores the pre-clamp viewport synchronously.
• Keeps retained collapse protection valid across subsequent geometry changes without creating per-event timers.
• Preserves user-scroll and non-clamp behavior.

Verification

• pnpm run type-check:web
• pnpm run lint:web
• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/FlowChatViewportCoordinator.test.ts src/flow_chat/components/modern/useFlowChatFollowOutput.test.tsx src/flow_chat/components/modern/VirtualMessageList.session-boundary.test.tsx
• Result: 68 tests passed
• git diff --cached --check

Reviewer Notes

• The late-shrink recovery is guarded by physical-bottom geometry, stable viewport height, active streaming tail follow, and absence of user scroll intent.
• FlowChat diagnostics remain controlled by the existing diagnostic switch.
• No Virtuoso fork or new per-item observers were introduced.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.