Replaces `use-stick-to-bottom` and per-row session rendering with
`@tanstack/react-virtual`, matching what Cursor uses.
Chat thread (`thread-virtualizer.tsx`):
- Natural-flow virtualization (padding spacers, not absolute items) so
`position: sticky` on the human bubble still resolves cleanly against
the scroller.
- Custom at-bottom anchor: pins when armed, disarms on user-driven
upward scroll, re-arms at bottom, jumps on session switch +
`thread.runStart`.
- Loading indicator and `--thread-last-message-clearance` move to a
real `[data-slot=aui_composer-clearance]` node; drops the brittle
`:nth-last-child(1 of …)` rule that can't fire reliably under
virtualization.
Sidebar (`virtual-session-list.tsx`):
- Flat agents list virtualizes at >=25 rows; pinned and
workspace-grouped paths stay direct-render.
- `SortableContext` keeps all IDs; only the window mounts; dnd-kit's
`setNodeRef` is merged with `virtualizer.measureElement` so rows
participate in both DnD hit-testing and TanStack measurement.
Drops `use-stick-to-bottom`. Streaming test gets a global
`offsetWidth/offsetHeight` stub so the virtualizer's viewport sizing
works in jsdom; the scroll-up-doesn't-pull-back invariant still passes.
Brooklyn Nicholson
64ab17182a