Goal

Make the session timeline scroll controller the single source of truth for scroll decisions so that upward reading, bottom follow, jump-to-latest, and resize-driven restore behavior no longer depend on three parallel owners that can race each other.

Concretely, when this change is done:

• the controller owns the decision state,
• history reading preserves anchor position instead of snapping back to latest,
• bottom-follow behavior is still preserved when the user is actually pinned to latest,
• and the current workaround logic can be retired from the auxiliary scroll helpers.

Scope

In scope:

• Consolidate scroll ownership around session-timeline-scroll-controller.ts.
• Move createAutoScroll toward a pure measurement + imperative-scroll helper with no independent follow / userScrolled state.
• Remove bottomFollowLock as a parallel decision owner from use-session-scroll-dock.ts.
• Preserve current timeline UX semantics while simplifying ownership.
• Add focused unit and E2E coverage for the consolidated behavior.

Out of scope:

• Any visual redesign of the conversation flow.
• Toolbar, bubble, tooltip, or trow styling work from slice 11b.1.
• Broader message rendering refactors unrelated to scroll ownership.
• Reopening the already-shipped minimum workaround in PR refactor(session): rewrite session view per W1 visual lock for slice 11b.1 #589 unless needed to land the consolidation safely.

Relevant files or context

Background:

• The session timeline currently has 3 parallel scroll owners that can race each other:
  1. createAutoScroll — ResizeObserver auto-snap-bottom with internal userScrolled bookkeeping
  2. use-session-scroll-dock.ts — bottomFollowLock with 3s expiry plus followBottom() scheduling
  3. session-timeline-scroll-controller.ts — high-level scroll intent state machine (following_latest, reading_history, restore_latest, etc.)

Recent regressions that motivated this follow-up:

• First regression: scrolling to a history position, then receiving a new turn, no longer auto-snapped correctly. The minimum patch suppressed createAutoScroll ResizeObserver behavior for 500ms after wheel-up.
• Second regression: scrolling to a position, then scrolling again, snapped the viewport back to bottom. The minimum patch changed controller observe behavior, widened gesture-based lock cancellation, and reordered message-timeline.tsx onScroll handling.

Likely files:

• packages/app/src/pages/session/session-timeline-scroll-controller.ts
• packages/app/src/pages/session/use-session-scroll-dock.ts
• packages/app/src/pages/session/use-session-timeline-interaction.ts
• packages/app/src/pages/session/message-timeline.tsx
• corresponding unit + E2E tests

Related context:

• This follow-up was intentionally split out from slice 11b.1 so the minimum fixes could ship without delaying the current visual rewrite PR.
• Related issue: [Feature] Rewrite packages/ui + packages/app visual layer against PawWork design system (incremental PR series) #440 conversation-flow rewrite planning
• Related issue: refactor(session): rewrite session view per W1 visual lock for slice 11b.1 #589 current slice PR context (not to be merged into this issue body as scope)

Verification

• Unit tests cover:
  • weak wheel or touch gesture transitions the controller into reading_history
  • content_resize during reading_history restores anchor rather than latest
  • submit / layout reset while returning to latest restores the latest position correctly
• E2E covers:
  • scroll up into history, receive a new turn, viewport stays at the history position
  • click Jump to Bottom, viewport returns to latest
• Manual verification confirms:
  • pinned-bottom behavior still follows latest
  • reading history no longer snaps back to bottom unexpectedly
  • nested scroll containers do not hijack main timeline state
• No parallel userScrolled state remains inside createAutoScroll
• No parallel bottomFollowLock remains as an independent owner

Execution mode

Agent should investigate and propose a plan first, then implement only after the plan is approved.