kandev/docs/plans/threads-multi-session-windowing/task-02-activate-task-columns-from-viewport.md

id, title, status, wave, depends_on, plan, requirements, acceptance_criteria, system_design

id	title	status	wave	depends_on	plan	requirements	acceptance_criteria	system_design
02-activate-task-columns-from-viewport	Activate task columns from the viewport	completed	2	01-publish-compact-session-attention	plan.md	REQ-PLATFORM-VIEWPORT-SESSION-DELIVERY-001 REQ-UI-THREADS-DECK-003	AC-PLATFORM-VIEWPORT-SESSION-DELIVERY-001.1 AC-PLATFORM-VIEWPORT-SESSION-DELIVERY-001.2 AC-PLATFORM-VIEWPORT-SESSION-DELIVERY-001.3 AC-PLATFORM-VIEWPORT-SESSION-DELIVERY-001.4 AC-PLATFORM-VIEWPORT-SESSION-DELIVERY-001.5 AC-PLATFORM-VIEWPORT-SESSION-DELIVERY-001.6 AC-UI-THREADS-DECK-003.1 AC-UI-THREADS-DECK-003.2 AC-UI-THREADS-DECK-003.3	../../specs/platform/system-design/viewport-bounded-session-delivery.md ../../specs/ui/system-design/threads-conversation-deck.md

Task 02: Activate task columns from the viewport

Summary

Keep stable lightweight task shells for the whole deck, but mount task-session lists and selected conversations only in their preload and detail windows. Prove the resource bound with 30 deterministic task shells.

In scope

• Add a controllable board-rooted IntersectionObserver activation hook.
• Derive desktop preload, desktop detail, and phone nearest-snap detail sets.
• Keep ThreadColumn shells, ordering, flexible widths, deep-link focus, and horizontal scroll geometry stable.
• Mount session-list adapters only in preload and conversations only in detail.
• Unmount offscreen conversation details so standard session registration cleanup can unsubscribe.
• Provide a conservative no-IntersectionObserver fallback that never activates every conversation.
• Add RED/GREEN component tests with 30 tasks, viewport entry/exit, adjacent preload, deep-link target, and phone snap ownership.

Out of scope

• Session selector UI, status icon changes, session selection policy, backend events, or full shell virtualization.

Acceptance

• Initial render with 30 tasks mounts 30 small shells but session lists only for the preload set and chats only for the detail set.
• Desktop entry and exit update chat ownership without changing column order or scroll position.
• Phone settles on one detail-active task even when a neighboring snap page is partially visible.

Verification

Start with a controllable observer test that fails because every thread mounts its conversation. Then run:

(cd apps && pnpm --filter @kandev/web test -- --run components/threads/threads-board.test.tsx components/threads/thread-column-activation.test.tsx)
(cd apps && pnpm --filter @kandev/web exec eslint components/threads/threads-board.tsx components/threads/thread-column.tsx components/threads/use-thread-column-activation.ts components/threads/threads-board.test.tsx components/threads/thread-column-activation.test.tsx)

Files likely touched

• apps/web/components/threads/threads-board.tsx
• apps/web/components/threads/thread-column.tsx
• apps/web/components/threads/thread-conversation.tsx
• apps/web/components/threads/use-thread-column-activation.ts
• apps/web/components/threads/threads-board.test.tsx
• apps/web/components/threads/thread-column-activation.test.tsx
• apps/web/components/threads/AGENTS.md

Dependencies

• Task 01 supplies compact status for later inactive session selectors. The activation code must not introduce a temporary transcript-based fallback.

Risks

• Observer cleanup during fast scroll must not leave a detached chat mounted.
• The deep-linked shell must exist before detail activation so scrollIntoView still works.
• Phone detail ownership must use the nearest snap target, not all partial intersections.

Parallelism

sequential

Results

Implemented and verified. Threads keeps all task shells mounted, preloads session membership for visible and adjacent columns, and mounts conversations only for the desktop visible set or the nearest mobile snap target. The controllable observer suite and exact component checks pass with zero ESLint warnings.