perf(desktop): virtualize unbounded lists and warm the emoji index

[wpfleger96]

Opening threads, channels, and menus fired the macOS busy cursor because the desktop renders unbounded heavy-Markdown lists synchronously (no virtualization existed anywhere in the app) and emoji-mart builds its search index synchronously when the reaction popover first opens. #1022 deferred only the main timeline and thread reply list, which left the freeze firing about half as often. This addresses the whole class.

Approach

One windowing primitive plus targeted treatments per surface, chosen by what each surface's rows require.

• VirtualizedList (@tanstack/react-virtual, new dep) windows lists whose rows have no in-DOM state. The contract supports an optional non-virtualized sticky-header slot and an optional externally-owned scroll container, so surfaces that share their scroll region with non-row siblings still migrate.
• content-visibility: auto skips offscreen layout and paint while keeping every row mounted. Used where in-DOM state must survive: deep-link querySelector targets, open <details>, and drag-and-drop reorder — windowing would unmount those before they could be reached.
• useDeferredValue defers a single large parse so the surrounding chrome commits first.

Changes

Surface	Treatment	Why
VirtualizedList	new primitive	Windowed render, dynamic row height, sticky-header slot, external scroll container
InboxListPane	windowed	Clean self-owned scroll list
PulseView (notes + agent-activity)	windowed	Two lists share one scroll container with a sticky composer; uses the sticky-header slot + scroll-margin offset
ForumView (post list)	windowed	External scroll path preserves data-scroll-restoration-id
RelayMembersSettingsCard	windowed	Single clean scroll region
ForumThreadPanel (reply list)	content-visibility	Deep-link locates the target reply via querySelector + scrollIntoView
AgentSessionTranscriptList	content-visibility	ThoughtItem open/closed state lives in a DOM <details>; list does not own its scroll container
MembersSidebar	content-visibility	One inner scroll container spans sticky section titles, a <details> archived section, and heterogeneous search-mode lists
CustomChannelSection	content-visibility	Rows are drag-sortable
ChannelCanvas	useDeferredValue	One large Markdown document parsed on open
EmojiPicker	warm init({ data }) at idle	Prebuilds the emoji-mart search index before first popover open; fixes every picker consumer from the shared component

The main timeline (MessageTimeline / useTimelineScrollManager) is intentionally untouched — it already has #1022's useDeferredValue and its scroll manager is deeply DOM-coupled (scrollToMessage deep-link, ResizeObserver autoscroll, lock/restore). Windowing it is a separate, higher-risk change.

[wpfleger96]

Desktop list virtualization — screenshot evidence

Captured from the seeded Playwright spec (desktop/tests/e2e/virtualization-screenshots.spec.ts), each shot assertion-gated and green in CI.

Pulse windowed feed — sticky composer while scrolled

PulseView windowed feed scrolled mid-list with the composer staying pinned at the top, proving the translateY(start − scrollMargin) offset holds under a non-zero scrollTop.

Forum deep-link to an offscreen reply

ForumThreadPanel deep-linking to the 25th reply — offscreen at open — scrolled into the viewport, confirming a never-painted contain-intrinsic-size row renders correctly when targeted.

Members sidebar — both sticky titles alive in search

MembersSidebar in search mode with both sticky section titles ("Members" and "Not in this channel") rendered simultaneously under content-visibility.

Drag-and-drop section reorder

Verified by the green dragTo assertion in the spec: section DOM order flips [Priority, Archive] → [Archive, Priority] with rows staying committed under content-visibility. No screenshot — the reorder is not visually distinguishable in a static frame, so the assertion carries the proof.