refactor(web-ui): extract flow chat scroll stability helpers - #1822

Summary

• Extract Flow Chat viewport reservation types and pure stability calculations from VirtualMessageList.tsx.
• Keep the React component module free of non-component runtime exports.
• Update focused tests to import the extracted helpers from their dedicated module.

Fixes #

Type and Areas

Type:

Refactor

Areas:

Web UI, Flow Chat, virtualized message list

Motivation / Impact

VirtualMessageList.tsx exported several non-component helper functions so they could be tested directly. This triggered eight react-refresh/only-export-components warnings and made the component module incompatible with React Fast Refresh expectations.

The helpers now live in a dedicated pure logic module. This is a behavior-preserving refactor with no direct user-facing change.

Verification

• pnpm run lint:web — passed with 0 warnings and 0 errors
• pnpm run type-check:web — passed
• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/VirtualMessageList.session-boundary.test.tsx src/flow_chat/components/modern/FlowChatViewportCoordinator.test.ts src/flow_chat/tool-cards/useToolCardHeightContract.test.tsx src/flow_chat/tool-cards/TodoWriteDisplay.test.tsx — 4 files and 38 tests passed
• git diff --check — passed

Reviewer Notes

This change only relocates existing reservation and scroll-stability calculations. VirtualMessageList and its tests consume the same functions through the new flowChatScrollStability.ts module.

No user-facing strings, locale resources, or public contracts were changed.

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.