Summary

• add a shared chat-list anchor resolver for sent-message positioning
• anchor newly sent optimistic messages in the web timeline and temporarily disable live-edge maintenance while anchored
• add focused coverage for anchor resolution plus browser-harness coverage for the anchor scroll props

Upstream

• adapts the web-safe chat scroll anchoring intent from upstream 24abab789
• intentionally avoids desktop/browser-simulation and mobile/native scrolling changes

Grok Composer 2.5 review

• initial Grok review blocked on maintainScrollAtEnd potentially fighting the sent-message anchor
• fixed by disabling maintainScrollAtEnd while anchorMessageId is active and adding browser-harness coverage for the scroll props
• second Grok review verdict: approve with suggestions; no blockers

Verification

• vp test apps/web/src/components/chat/MessagesTimeline.test.tsx apps/web/src/components/chat/MessagesTimeline.logic.test.ts packages/shared/src/chatList.test.ts
• vp run typecheck
• vp check

Notes

• vp test --project browser src/components/chat/MessagesTimeline.browser.tsx from apps/web could not run locally because Playwright Chromium is not installed in this environment (chrome-headless-shell missing). The browser spec typechecks and is included for environments with Playwright browsers installed.
• Existing unrelated warnings remain: ProcessResourceMonitor.ts Effect suggestion during typecheck and pre-existing react(no-unstable-nested-components) warnings during vp check.